WotLK look likes Blizzard took every complaint they had ever gotten and figured out how to address them, without caving in to whining.
Server stability seems to have been ruined and then fixed with the release and week or two after 3.0. That week or two was terrible, but I'm glad to have had the problems worked out then instead of now. My only complain with the servers is the queues, but that's partially due to players, both returning people and those who are on anyway playing a lot more. I imagine this will level out a bit as the numbers of 80s go up and the leveling rush slows.
Hellfire Peninsula was terrible early on. It was incredibly crowded and mobs were a pain to get. Variable respawns helped, except then you could have a mob instantly respawn on top of you. Or ten. Dead you. Now there are two starting zones. This cuts the zerg in half. To make it even better, there are scattered quest hubs, so people are spread out more. If one area is full, move somewhere else. The final big fix was with many quest mobs being summoned, eliminating the rush to tag mobs.
Questing is set up better as well. In the past quest stacking was unnatural. It required waiting around for all quests to become available for an area, often running to different quest hubs to get them all, and sometimes having to finish long chains first. Now there seems to be a short chain to introduce an area, and them up pop 4-5 quest markers for a single area.
Zones are more contained, so you can finish one and move on to the next in an orderly manner. This is great for completionists. This is also great for reducing the irritating travel times of the past. But the zones aren't totally separate. When you're done with an area, you get quests that send you to the next. It's organized, but if you're not looking for it you don't see it, you just see that you're having fun and not getting lost and constantly back-tracking.
The instances aren't so hallway-like. Azjol'nerub was really cool, which made it unfortunate that it's so short. Apparently there's another wing, which I'm looking forward to.