*shrug* I dunno, CoX is the only MMO I actually enjoy. Lots of customization options available with builds, everyone doesn't look related, proper slotting of IOs can make some insanely powerful characters, and MA opened up a lot of creativity options (boss farms don't work anymore).
Honestly however, the thing I like most about it is you don't need the "holy trinity" (tank, dps, healz) to do anything so it's very much just log on and play, without worrying about having to wait 40 minutes for the shaman or enchanter or cleric or whatever to log on. I also like the fact I can set the game down for 3 months and come back and my toons aren't out of date and so far behind my guildies I can't play with them anymore. This MMO is just very friendly to those of us who don't want to spend more than an hour or two every third day in front of the computer.
I realize this isn't the majority opinion of MMO players, but that's fine there's plenty of other MMOs out there to satisfy those players. CoX works well for me.