Living, breathing, immersive fantasy (fantasy meaning not real, not meaning SWARDS AND DRAGOONS).
Non-repetitive NPCs. Economy affected by player trades, etc. Lots of random events to keep the world fresh. Environment that's able to be...molded by player actions (read, If I chop into a tree, that chop'll still be there, things of that nature). Maybe some combat based on player skills, not on "paper skills" (think like a fighting game relies on player skills and your average RPG relies on paper skills, stats, etc. and the like).
All of these are really technically far off, but it's still what I'd like to see.
Edit: what should be removed? Zoned worlds, such as FFXI, RO, etc. Not one big continuous world, but sectioned up. Totally takes away from the immersion.
Also, the world must be fully able (within transportation limits) to be traversed by the player. No invisi-walls, no huge barriers of walls everywhere.