I have an account on AION, WOW, and FFXI. The only one I still play on a regular basis is FFXI (even though my FF friends play aion now >.>). Here is my reasoning.
WoW: I've played it for years, it's gotten old for me. Most of my friends I played with don't play anymore, or are scattered on different servers. I've seen the content, and really have no desire to play the game for content alone anymore. I don't like where the game is heading, they've made everything too easy. It's ridiculous how easy it is to get end game gear. It used to be an accomplishment to get an epic piece of gear, now it's a joke.
Aion: It was very fun in the beginning, as it is different than WoW in ways. The game isn't really that bad, just not to my taste. There is a big emphasis on PvP, and I really do not enjoy PvP. The crafting system is kinda cool, as is the gathering system. One thing that kinda annoys me (this isn't major, just slightly irritating) is the fact that they seem to put big numbers in this game just to say there are big numbers. For example, it takes over 42 MILLION experience points to get from level 41 to level 42. That's right, more than 42,000,000 experience points. Is that really necessary? No. The currency is the same way. At low levels it isn't too bad, but eventually you're spending millions of kinah (gil) like it's nothing. On the plus side, Aion is more challenging than WoW. (So is eating breakfast or tying your shoes).
FFXI: Of the other MMO's I've played, this one is simply different. WoW and Aion have a similar feel to them. FFXI still offers a challenge, and the job system is something I enjoy. Sure, it's not as action packed as the other two, but sometimes I prefer that. Do I miss being able to jump? Absolutely. I enjoy all 3 of said MMO's in my post, but I just find myself having more fun in FF. All personal preference :)
Sorry for the long winded post.