I have no doubt there are other good MMOs out there, RIFT is probably the pick of the bunch but I have no interest in:
a) getting hooked into another MMO.... ever again. They are far too time consuming.
b) leaving WoW for another MMO. In the past I have tried AION and Warhammer and both are just WoW without my existing social group. It is this social bond that keeps people playing wow and no matter how good another MMO is, as long as WoW holds the social group cards people will not change.
just my 2cents