I have played Conan, Warhammer and Aion. I am now giving LotrO a spin. I really liked Wow but then at some point it seemed to become a bit too much for me. I have picked up my account again after a year but seem to have a hard time getting back into it. Wow is very content rich - probably much more so than anything else I have seen.
Conan was a rip-off as it started. I bought it because of it suppoded spectacular DX10 looks (which were not in the game as they immediately pulled the DX10 engine). Game play after the first handful of levels (3which were extremely well done) was so bad, it felt like you were in a different game. Eventually I stopped playing because I had no interest that post lvl 20 game vs. the initial great start. I think players have never forgiven the company for that bait and switch.
Warhammer is very nicely done, not quite as cartoonish and relatively rich in content. It relies quite a bit on a proper server population as the game favors collaboration. PvE is nicely done. Like Conan you cannot really say it is a WoW clone; it has the same elements, leveling, trade and crafting, etc. but has its own feel and character.
Aion on the other hand is a complete WoW clone in most every respect (Yes, you can fight while you fly - as if that made a difference). The cloning goes so far as that the whole look and feel is almost identical. It seems as if whole landscapes were lifted. The sound track, IMHO, is pretty bad and reminiscent of samples slapped together with Acid (a piece of software). I could swear that the combat 'rock guitar' sound is straight from one of the Acid loops. Aion also has a decidedly Asian ambience. While the combat sounds and voices are of course supposed to be language independent, they sound like Pokemon to me.
Aion seriously lacks content and is predicated on the idea that you like grinding. Leveling crafting involves standing in a room and essentially doing the same thing for however many hours you like. It has the worst crafting system of any game I have ever played. Gathering is similar - leveling is intentionally very slow (chances of gaining a level increase in gathering are very low). The game is overrun by gold sellers and, since there is hardly another way of leveling, bots. Now that NCSoft has gone through a round of bans and mods to the game to get a handle on those I am certain many players will leave due to the sheer lack of genuine content - something that has taken Wow (and I have played Wow almost since when it came out) a long, long, time. Personally, I do not enjoy quests like 'Obtain 25 shards of this and kill 30 mobs of that kind'. It's a rip-off to simply force you to spend time in the game. This is what Aion mostly does. As Wow grew it became more of a 'Steal this and then smuggle this and then discover that' - much more challenging and interesting than 'Grind a gazillion of these'.
So as I am pondering my re-entry to Wow I am giving LotrO a whirl - I have heard a lot of good things about it. From my experience - if you liked Wow you really might like Warhammer. It's similar but not too similar, the content and game play much more interesting than Aion. Aion is occupational therapy at best.
I shalll give fallen a try myself as well.