For me what appeals about MMO's is that if I buy a game for single player I beat it in 1-4 days and guess what, there goes $60 and your looking for the next one. Multiplayer usually stagnates after maybe the first week or so, especially when you can't find a good group of players online that arn't tards. If your lucky and you do the game sticks with you for a a while. I my case C&C Renegade, CS, Sins of a Solar Empire, Swat4
For a MMO the biggest key for me is that there has to be something new with players you enjoy hanging around with. I've played WoW and a few others but EVE caught my attention. When I first started playing I joined a indy based corp, so I've done all the mining stuff found it boring as hell. Though shortly after I joined we got war dec'ed by a Merc corp, which force fed us PVP, we adapted and grew. Later on our corp joined an alliance.
So pretty much MMO's for me personally need 2 things, a good group of guys to play with, and something changing in the world, perferably player run.
Immagine if in WoW they turned all the contested area's into a zone control map. Where you can hold different check points in the world, and so lvl 70s don't just whore all of them, make some area's that only low level charecters can capture. Bring some teamwork, territory defined real world PVP, make it accessible to n00bs, This will get newer players working with older players (though there will be a shed load of twinks, there isnt going to be much your able to do about that.)