See.... with me, I see the DM as a
referee and a person that sets up situations and locations for the PCs to interact with.
They aren't there to fight the players; the DM is there to know the rules and make sure that everyone gets a fair shake at playing the game.
Seriously, these would be some of the rules that every DM should follow, in fact, every player should follow:
1) PCs are meant to survive. Doing things that is counter to this idea is a
bad idea.
((This means no turning
every fucking dungeon into an insane war-fortresses that are run like real fortresses and can stand up to armies, let alone adventurers. I made this rule up because I can
design impossible to beat war-fortresses, I seriously spend time and sit down with
other DMs and have designed dungeons that use the special abilities of the monsters that live in there in order to create dungeons that are very difficult to assault, even with a massive army of NPCs the same level as the PCs.))
It was a "mansion" controlled by a very organized group of vampires that basically controlled a small nation.
The attackers were werewolves that had arrived recently, and had decided to take over the place. Werewolves can be active at night and day, the vampires only at night. So the wolves had, over the course of a few montsh, driven the vamps to their house. I helped the DM design the house.
The prime ability used is the fact that vampires have Gaseous Form. So the place has tons of areas that are only accessible via long and small shafts. Allowing the vampires to have free reign of the place, while no one else can possibly threaten them.
In fact, the whole top floor, where the vampires live, is impossible to be accessed except via gaseous form. Making the taking of the place really hard to do. The wolves would need to make piles of furniture to reach the very high ceiling, and then try to break a hole in the ceiling in order to get to the top floor.
Even things like the "main entrance" are traps. There is no main entrance, the main entrace doors lead to a giant room with no doors and a surrounding balcony. The main gate can be shut with a portcullis and then mere crossbowmen can fire at the trapped enemies below. The servants come in through smaller doors in more isolated areas of the mansion, and the vampires often just come in through the windows.
Those two elements alone could reduce any "invasion" by half or maybe even completely wipe out an invasion. The main gate "death hall" was meant to accommodate 100 people easily
2) The DM is playing Monsters or NPCs. The DM is
NOT playing characters that they enjoy or will get attached to. Any character that the DM gets attached to
must immediately be kidnapped, killed, vanish etc. NO FUCKING EXCEPTIONS EVER.
The PCs can kill NPCs that you feel you are getting attached to. Do this immediately, and let the PCs savor their victory.
This is the only way to prevent DMs from having NPCs have too much dominance over the story.
The only time that an NPC can be attached to a character is if the character will only help the PCs, and in a "I'm assisting you in acheiveing your goals" sort of way, not "I'm going to save the world now for you guys" sort of way. Combat NPCs that are more powerful than PCs, or spellcasters that are higher level are bad. A healer that is higher level than the PCs is much more acceptable for an NPC.
3) Never fucking shove your story down the player's throats.
Your story is
shit. Your
story is shit. Your. Story. Is. Shit.
Do you get me?
Do you fucking get me?
You are a piece of shit writer and you cannot get any of your shitty ideas published. Therefore you should not try to ram down your stupid story down the player's throats. At most, you should present a setting and have the players interact with it, the setting shouldn't force the players around unless the players have actually been doing things for a long time and actually affecting your setting in such a way that something will now seek them out.
I think that there might be one person ever that I would allow to railroad me. Natch, make that two. Two people that I have met in my entire life that I would be "okay" with being able to railroad me. One will probably never DM, ever. The other I will probably never meet in person, let alone sit down at a table and game with them, or even game with them online.
If you force PCs into bad stories, the ghost of RPGs past will come after you and beat you to death with a railroad tie. I fucking kid you not.
[Note: these two people that I'm referring to are both 1) excellent writers, and 2) I actually trust them a great deal, one based on what they have written, the other based on my experience of them as a person. If you are reading this, you are probably
not one of those two people.]
If you aren't an anthropomorphic creature that can take on dozens, if not hundreds or even thousands, of different fuzzy animal forms or birds, you aren't one of these people. If you aren't a brutally honest regenerating-monster-human-male, then you're not the second.
4) Make sure that everyone is on the same page.
If a player does something that doesn't make sense, inform them about what their character should know. A lot of DMs suck at describing, a lot of players also suck at understanding the drivel that a DM is saying.
Make sure that everyone realizes that there's a visible pit in front of the door, before anyone charges into the room. If someone does anyway, make sure that they are reminded of the pit.
5) Powergame, and min-max.
First for the players. If you need, for your monsters and NPCs. Seriously, do this. Everyone will have more fun, and your group will have much better stories to tell afterward.
Always tell a player as many of their character's possible options.
Especially if you are a stronger player or know one of the weaknesses of your own encounters that a PC might take advantage of.
The only reason that I can run the encounters that I do, and not have my players utterly loathe me is that I give suggestions on how to use their abilities to get the maximum benefit out of them. I've got a bit of a tactical bent, and after playing some turn-based tactical games with people that knew what they were doing, I learned how to approach turn-based combat more effectively.
When the DM is willing to help players Min-Max and powergame, then can a game really be played. For the DM it's exciting to watch a lone monk go after a Catoblepas in a swamp, or a group of PCs gang up against a single very high level challenge and not only have no PC die, but also kill the monster. For the players, they get a really good story to tell afterwards; so do you as a DM.
There's probably some more, but those are the ones that really stick out for me.