Nathrezim of the Karazhan server posted this question today on the European WoW forums: what don't you want to see in WotLK? He threw in one limitation: no complaining about class balance or skills. Focus on quests/instances/mobs/mechanics.
That being said, some interesting comments so far include: no caster swords as faction rewards; no long runs from graveyards; and, of course, no more of the universally hated oozes.
What game element do you hope will be left behind with the new expansion?