A better question would be what would an MMO need to keep you playing.
Meaningful pvp and sufficiently interesting small group content would do it. Rift blew it primarily in the former, though the small group content got stale after running T2's for the 1000th time. I love the soul system, enjoyed the combat mechanics, and found the classes to be quite interesting. Rift is a very well crafted game. Were I more of a raider and pve enthusiast, I'm sure I'd still be playing it.
"Always do what is right. It will gratify half of mankind and astound the other."