Setting: Star Wars or Warcraft. They both have brilliant settings and I like the idea of playing a Druid (watched the WC3 Medivh cinematic today - goddamn if that's not the most awesome class ever) or a Jedi/Sith. Other settings would work too, but these are my top two.
Story: At TOR level or higher, both in quality and deliverance. We're way past the point technologically where quest text is the acceptable means to delivering the entire story.
World design: Something as or more open than WoW. 3D movement (diving/flying) and zones are open, not a linear shoehorn sort of design TOR has going.
Combat: Willing to accept standard hotkey MMO combat, but if we're going for perfect, take a page out of Jedi Knight (Outcast/Academy) or TES - FPS/TPS for gun/bow using classes, first (TES)/third (JK) person action game for melee characters. I'm cool with the trinity as I think it creates a sense of teamwork, but healing needs to be looked at a little so healers don't spend their time looking at the UI instead of the game. This would probably be bettered with a non-hotkey system to begin with though.
End game: Let's start with the basics, raiding - there needs to be something for large groups of people to do at end game, so the raiding concept stands. However, it needs to be more about progression and less about the loot treadmill. For one, I'd like to see a progression model similar to what WoW: TBC used - moving from 5 mans to 10 mans to 25 mans, with attunements and a story in between. 5 mans and 10 mans shouldn't JUST be a stepping stone however, they need to be a thing of their own, but the progression should stay there, at least at the beginning (first set of raids), to both introduce players to working with larger and larger groups progressively and promote a sense of achievement.
The raid model should have a difficulty curve starting relatively easy and getting harder over time. It should strive to always have more content on the horizon for most players as opposed to getting each player through every bit of content. It should not have different settings for either size or difficulty.
That said, it must, I repeat MUST not be a "raid or die" model. This MMO mentality is dying out and every MMO that comes down to it WILL fail, including WoW itself (I don't mean fail as in shut down completely, but it's not gonna keep as many people for much longer). There needs to be stuff to do at maximum level and the more diverse things are, the better. Moreover, these endgame aspects need to get the same level of attention that instances and raids do now in something like WoW ie. they need to be updated with every patch.
Dynamic content: A fantastic way to introduce this non-raid endgame stuff is dynamic content, but it needs to not be as stale as something like Rift. Every sort of content will be repetitive after a while if not updated, but something needs to be done to make it interesting and dynamic day in and day out.
PvP: I'm not a big PvP person, but a few quick thoughts. First off, what I'd care about the most is some kind of PvPvE. Mixing PvP and PvE ends in fantastic things and if you throw some dynamic content in there, done right, I think it could be one of the major things to do at maximum level for both those who like PvP and those who like PvE.
Also, Arenas. Again, not a big PvP person, but I think Guild Wars had the perfect PvP model, certainly the best I've seen for these kind of things. Guild vs Guild is also a good idea. There also need to be large instanced battlegrounds.
Social: This adds to the diversity of stuff to do at maximum level. Minigames, social items, events etc. There also needs to be a social sense for each of the aspects of the game. For example, in PvP, guild alliances and wars would be a good idea, Guild Wars had both, though something more dynamic that could happen out in the world as opposed to being closed off in private areas would be better.