ive been a stoic WoW'er since it was released and it is the best out there by a long, long way - unfortunately.
with that said, i tried Age of Conan and Warhammer Online when they were respectively released. AoC didnt last me very long because it was just...bleh, where do i start.
The loading screens between all the different zones make it feel like a bunch of individual zones rather than an actual world.
The lore of conan is nowhere near as wellknown or mainsteam as that of Warhammer or Warcraft, which undermines what you can actually understand while playing.
After the first 20 minutes (which are great) there isnt much feeling of progression...you are suddenly thrust out into the big wide world without knowing who to talk to, where to go etc. The fact that you have to travel/load between zones really underpins this.
As for WAR, it was more promising I felt. The main problems was that it was simply so buggy. Unfortunately, with WoW dominating the market to such an extent, why settle for a buggy WAR when WoW has already had years to iron out and perfect those bugs?
Finally, if you want something a bit different, I have heard a lot of good things about EVE Online. Very steep learning curve from what I gather, but if you dont mind it being about spaceships and sci-fi then it sounds like a pretty good bet. Pretty much everything in the game is player-built, from the ships to the factories to the space stations. Player controlled guilds can hand out "quests" to the players (like, go mine 10 of this ore we need, or take this cargo from X to Y). If you go mine that 10 ore, the guild you hand it in to is going to use that to build part of some big missile launcher, which in turn is going to be sold on by the guild for a profit, or attached to one of their ships.
Its like a real economy, not the NPCs with an infinite supply of gold, handing out the same "collect 5 wolf tails" to all players.