To this I would add:PoliteNewb wrote: So maybe this "high-level" definition I'm seeing comes down to:
--High level people never travel on the ground...they fly, or teleport, or astrally project, or have a spaceship. So you can (almost) never come to grips with them.
--High level people use ranged attacks almost exclusively, usually highly devastating ones. They shoot lasers, or explosions, or use ki-fist-dim-mak waves or something.
--High level people don't worry much about the mundane planet...they're busy with other planets, or dimensions, or whatever.
--High level people ignore mundane dangers. Things like fire, ice, hunger/thirst, sleep, terrain, poisons, whatever...none of these things matter, unless they are magical (and usually high level magic).
--High level people consider death an inconvenience at worst; to fully deal with them, you need to trap their soul or something.
--High level people receive deference.
--High level people perform favors or fulfill requests, they don't take jobs or follow orders.
--High level people come up with the plan, and assign the duties.
--High level people are not trying to stay on your good side, you are trying to stay on theirs.
--High level people are not vulnerable to the machinations of people a certain distance below them.
--High level people get involved with things that are neither simple, straightforward, or cut and dry.
--High level people sometimes change things in big ways. If you don't have to make a note in the campaign guide about how things are different now in a way that would be noticeable and profound to people outside a small geographic sphere, then it might not have been a truly high level adventure. There's a difference between an adventure being high level because it involves a CR17 dragon and an adventure being high level because you changed the elemental affiliation of efreeti.
A high level adventure might, depending on your setting conceits, involve something like the denizens of a demiplane enticing the party to grant them an audience so they might relate their plight of the githyanki assault on their corner of the cosmos. They humbly beseech the party to drive out the githyanki invaders and destroy any knowledge the githyanki have relating to their demiplane (location, resources, existence, etc). All while attempting to keep secret any unique qualities or features that would inspire the party to conquer the demiplane for themselves.
The PCs would decide how to go about doing that and would essentially be "in charge" the entire time. Though their enemies might number in the thousands--figure the Githyanki assigned a division to this conquest--there are going to be conflicts that are resolved by simply handwaving a company of githyanki soldiers out of existence (because 13th level Druid > 200 githyanki grunts). They still have to deal with the larger issue of making the upper command structure "forget" about the demiplane (and the party, if they're concerned) and the potential decision to keep it for themselves. Even said, that "high level" adventure might be for 11th - 15th level characters.
Even after this adventure is over, there's probably going to be continuing concerns about the githyanki, making use of or governing the demiplane, or both. Maybe the demiplane will serve as the jumping off point for the next adventure where they attempt to civilize and ennoble the goblin race by usurping the cruel and sadistic pantheon that keeps producing them as cowardly, skulking scavengers. (This one might be more truly high level, as at the end of it your characters might be overseeing the birth of the goblin Martin Luther King, Jr. and the MC is line-editing the goblin entry in the monster manual to Alignment: Any and removing the Charisma penalty.)