Okay, I'm going to give it a shot, though I'm not sure I'm the right person to do this. I'm a long-time Earthdawn player, occasionally GM, and a big fan, though not as big as I were once (there was no new material for a while, so I grew bored with the game a bit). And I apologize if this post seems a bit incoherent. I didn't get much sleep, and the pain meds are messing with my head again. I'll try also to make this more clear to people who don't know much about Earthdawn.
First thing you should know is, that Second Edition is a piece of crap, game mechanics and storyline-wise. The 'a great dragon did it' excuse was way overused in the storyline, and the mechanics were even more needlessly complicated than in first edition. And don't even get me started on the details, because I'd ramble on till morning.
Basically, Earthdawn was remade by RedBrick Ltd., which consists of a group of hardcore system fans. Imagine Frank and Ancient gathering the old freelancer-veterans and remaking Shadowrun as Shadowrun Classic. It was kinda like that with Earthdawn Classic. It was First Edition with a lot of patches, fixes, and overall improvements, gathered in only a few huge sourcebooks, instead of dozens, heavily indexed and organized. And it worked. I know, because we started using it in our group as soon as it was released. And it worked wonders, comparing to the First Edition.
But it still had flaws that could be considered fundamental. Like the 'difficulty modifies step' rules, which messed things a lot. So RP decided to release Third Edition, and they did a pretty good job. First thing ED3 did right was getting rid of d4 and d20 from the Step system. That saved many people a lot of nerves, GM's and players alike. Second was fixing the step system by changing the way bonuses/penalties work. Instead of modifying the step number you add/substract them from the result. That fixed a lot of things, including combat mechanics. Other than that ED3 included a nice rebuild of the Discipline system (finally you have a choice which Talents to get), and hundreds of little fixes and clarifications for everything, from the magic items, to specific talents/skills/spells. And it works. We switched to ED3 right away (again) and had tons of fun with it.
Off course there were blunders, like the infamous falcon entry in the GM rulebook bestiary. The reason one single falcon can kill almost any first circle character could be explained as 'those buggers survived the magical daemon apocalypse', but still...
Earthdawn Third Edition isn't perfect. It's still doing a good job when it comes to fixing an old game system.
That's the mechanics. The fluff part stayed as it were in ED1 and was capped at Prelude to War, a storyline-driving adventure that made Barsaive a lot more interesting. So forget any ED2 storyline crap you might have heard. Vasdenjas is still kicking, and there are no unstoppable-horror-locusts swarming the land.
What Earthdawn has, that DnD (at least 3.x and 4.x) never had is the way all rulebooks are written. Everything that isn't game mechanics or gamemaster's guidelines was written as if it were a chapter from one of the books written by dwarven scholars (it's a real Discipline - being a wandering wise-ass scribe :P) from the Great Library of Throal. Like the book of Horrors for example - written as a study of those outworldish cthulhu-esque entities. There's even an instance where one of the scribes used the name of one particular Horror too much, and he ended like a classical HPL character. Like this:
(...)
Dandeer’s account ends here. Her attendants at a local shrine to
Garlen add a note that Dandeer can now walk well on her wooden
leg. Dandeer and other writers accept implicitly that Nebis can
notice and manipulate unwary Namegivers who invoke its Name
too often. Your humble scribe finds this unlikely, for Nebis Nebis
has not demonstrated clairvoyant powers of this Nebis description,
certainly not on those Nebis whom Nebis has not Nebis
already marked. Assuming such Nebis were true, the effects of
Nebis’s Nebis’s NEBIS’S manipulation would become evident
to an NEBIS observer. NEBIS To date no such NEBIS has surfaced,
despite the NEBIS diligent inquiry of one NEBIS who only
desires to NEBIS the NEBIS of the NEBIS Library of NEBIS,
and hopes that NEBIS superiors will NEBIS him with NEBIS
and NEBIS NEBIS NEBIS.
Enwid Deltern, 1445–1507 TH. Lochost preserve his memory.
—Merrox, Master of the Hall of R ecords
I admit, it might not seem as something really original, but when every single chapter of a rulebook aims to do that (even those that are filled with rules have some fluff quotes and introductions), you start to treat it as something natural. That is probably why I like Shadowrun so much. I know that's probably 4th world version of Shadowtalk, but it's still something. I've only seen it in really neat games, like Eclipse Phase, or the old CP 2020 for example. I could go on like that, but it would be only my opinion and point of view, and you need to make your own. ED is an under-appreciated game that has a lot of potential and no 'right way' of playing it. And it still*(?) has immortal elves and great dragons if you like them, but you're not forced by the storyline to set them up around every corner. Dunkelzahn/Mountainshadow is already warming up to metahumanity (Namegivers, as they are called), Ghostwalker/Icewing is still a manipulative dick, but they fit in the game nicely.
As for the list of things to fix in DnD:
Get rid of health/damage asymmetry.
You don't have to. Healing is much more common, and the damage system a lot more random. You can oneshot a dragon with a (magic) dagger, and it fits the game theme. You'd just have to roll over 250 on a 2d10 roll (off course the dragon's fate manipulation power will force you to reroll it, but still, you could kill that dragon with a dagger, because only non-magical damage is capped stepx3)... An average character has 3 recovery tests per day, rolling his Toughness step (Death/Unconsciousness thresholds are based on this, obviously) and removing damage according to the result. That, plus a ton of possible sources of healing test bonuses (spells, potions, herbs, skills, talents, and so on) and Wounds, that impart a penalty that applies also to healing makes up a health system that works well for a heroic, apocalyptic high-magic fantasy setting.
The high-level effects will actually have to reflect the reality of high-level.
There is no game core anymore. Just tiers, like DnD4 has, but it has taken them from ED, I think. Basically, you have Initiate (1st circle), Novice (2-4), Journeyman (5-8), Master (9-12) and Grandmaster (13-15) circles ('levels', but ones that are used in-game, though it's rude to do that to someone). And the endgame Talents are really something, like casting multiple spells as one, making up to 15 attacks per round, walking the (corrupted and haunted by Horrors) Astral, and so on. The spells at Master and Grandmaster level are legendary also, but in a good way. A Ressurection spell that leads the character's soul back to his body, and takes as many days to cast, as he was dead for. A fake, illusionary eclipse or sun, or a whole city in a bottle are good examples. There is no real threshold, though. You get better with each Circle and Talent level.
A decent multiclassing system.
It's there. You can take as many Disciplines as you want. There are a few 'buts', like the cost of talents will be higher with each discipline (the fourth will have all of it's talents as expensive as the engdame ones), or that you have to somehow cobble together the philosophy (yes!) of a Thief that goes by on deception and deceit, and a honorable, thruthfull Weaponsmith without making your character suffer an existential crisis (am I a thief that smiths weapons, or a weaponsmith that thieves?) that takes away some of his magic. Also, you need a teacher, a lot of time, and money (the instruction costs are exorbitant for a low-circle character). Or you can play a Human. Humans have the Versatility talent, that lets them learn talents from other disciplines (up to the Versatility level in both number and max level).
Stronghold Rules in core rules.
None, but you can claim one and Name it, giving you neat bonuses whenever you're there. The same can be done with the party - you create a Group, bound by a magical oath that lets you strengthen each other through magic. For example by raising your Durability talent (the 'bonus hit points' factor in Earthdawn) of your Wizard companion, so he won't die in a fight... again.
There are some rules that can be used for it, and guidelines to fill the rest. You can get your own river/airship though, with all the benefits and drawbacks (and easy-to-use ship battle system). And it sure has a name too, so you can also use it for your own benefit.
A decent default campaign setting.
It is. A few scattered, but influential city-states, one dwarven kingdom, one corrupted elven forest, one oppressive evil slaver empire fueled by magic. All of that in a world raising from the ashes of a deamon apocalypse, that isn't quite over. We've been playing for years, and some elements of the setting weren't introduced yet. But you can easily fit in your ideas, add a pinch of Barsavian culture and lore, and voilla. And you can always say 'it's ancient magic from before the Long Night', and put it in a Kaer or Citadel (they're basically dungeon crawls inclueded in the setting). Recently added was the Cathay rulebook, which was 'oriental adventured', only based off Chinese warring states era, and way better than the DnD one (which isn't really difficult). It even has it own novel series.
Actual save-or-sucks need to be re-implemented.
Most spells and abilities are hit/miss, but some need to be overcome by a Willpower roll, especially in case of creature and Horror abilities. You can have talents that allow you to 'dodge' spells, as with the basic dodge rolls. The same with social abilities, like taunt, or challenge. So, save-or-suck factor is there, and it's the right kind of ugly. For example, why are the mid-range Horrors so deadly? They can roll ~4d12 on a fear-inducing Terrror power, and you have to beat their result with your Willpower (usually d10) before you can do anything but soil yourself in a corner. That, and the power that lets them rip your skin of the body and turn it around for fun and breakfast (they do feed on pain, fear, and revulsion).
Expand, standardize, and fix the skill system.
There are plenty of skills to be taken, and even more Talents (the magical Discipline-based version). You can take your pick, and have fun jumping of flying airships to glide safely down, make your own blood boil while it's flowing from the wound, sealing it, or have an elemental pick a safe path and protect you while you walk on it.
A treasure accumulation system that is not ass.
Magical Items are uncommon, powerful and unique, and you can have only a finite number 'attuned' to you (and even that non-magician Disciplines cannot do until fourth circle), capped by the Threadweaving talent, which is also used that way for other Thread magic, like group and place attunement. Even if you buy a 'generic magic sword', it has its own name and probably history. If you use it a lot, and do great things, it's magic will be strengthened. And there's a ton of general magical treasure, like coldproof cloaks, magical zippo wands, and cauldron you can put on someone's head and boil it... Well, you can't do that last one, but it would be fun. :P
And there are more powerful magical items, at least twice as good as the general ones, but not unique, and artifacts, totally out of their league and one of a kind. Most of the examples in rulebooks are really nice, providing ideas for stories and even whole campaigns. Any magical item is also a plot device - you have to learn it's history to use it, and as you advance in ranks the knowledge gets harder to find and more dangerous to obtain.
Also, some treasures are just an excuse to brag about (look! a wyvern horn! *ding, you get XP for showing it off, proving how much of a hero you are for slaying one*), and then sell it. 'Harvesting slain monsters for XP and profit.'
A core rules class and powers system that is simultaneously more robust and more efficient.
It's consistent, robust and customizable at the same time. Complemented by special abilities, increases in Defense, and so on. As you get higher circles, there's even more of those. You even get minor magic abilities, that are mostly quite flexible (an Archer can make his own arrows, a Wizard can sense magic auras, a Sky Raider can use it to fix his airship, and so on). And yes, it's efficient. It makes you look at other games with disappointment, because their 'classes' are hollow pieces of game mechanics. In Earthdawn the Discipline is a way of life first, class second, or even third. And the world's magic system is at it's core - there's power in the Discipline magic, because people follow it, and the more of them do, the more power there is. In the old editions there were some Disciplines that had only eight, or twelve circles, because there weren't many Adepts following it, and thus the Discipline in question didn't have as much power.
Core classes which feel meaningfully different from each other.
They are. As I mentioned in the paragraph above, eac Discipline is a mix of philosophy and way of life as much as means of magical power. If you can find your own path that doesn't contradict it's core ideals, you can easily implement it in the path. Adepts are also champions of the common people, being natural born (or trained) heroes who fight the remnants of the Scourge, bandits, ork scorchers, evil theran slavers, wild beasts, and so on.
They're mostly compatible with each other, but there is none that makes you feel that it's redundant. Even with two kind of sky-roving disciplines, Sky Raider and Sky Sailor, there's a huge difference. Yes, both work best on airships, but the former is basically a wild viking-pirate-berserker, and the latter a civilized, swashbuckling, dashing swordsman from 'The Crimson Pirate', only his ship flies.
Even with different kinds of spellcasters that works wonders: Nethermancers are not necromancers - they're obsessed not with death, but with other planes of existence, and that's where all kinds of spirits come from (Horrors included). Wizards are 'book mages', but they work with Ideas and Ideals, like those in Plato's works. They take things apart by means of logic and magic, and find the core principle. Elementalists work with the five (4+wood) elements of creation, which have its own impact and relations with each other, and which create the world around. Also with elemental spirits, and the idea of balance. They're like druids who worship the elements, and nature only as the manifestation of elemental balance. Illusionists are showmasters, obsessed with the idea of world as a stage, and with truth as an ideal. They can use trickery, but not in a way that hurts the truth.
* I know ED is a 'prequel', but that would take into account the fact that SR was first.
And to answer Lago's question: because most of the thing you'd like DnD5 to have Earthdawn has already. And it's available, fun, and won't be screwed over by 'gaming gurus' like those you mention in the DnD thread, because the people of RedBrick are old-time hardcore fans. :P