The heart of the matter that people don't seem to want to give up is that the power of the Jedi relative to other classes has to be balanced. "Everyone," even people who want an MMO to work, still want their uber Jedi. The questsion is whether the developer is willing to ignore the screams of bloody murder for the long-term viability that balance between classes would bring.
Incidentally, the Jedi being the best warriors, wizards, and possibly even healers is a characteristic of the Star Wars setting as a whole that hurts multi-player gaming, and yet the opportunity to be a Jedi is what draws people to role-play in the setting. It's self-defeating on that front. Yet, with the willingness to make sacrifices regarding a Jedi's powers and endure the outrage that accompanies that choice, I think it can be done.
I might argue that a post-SPRPG setting actually promotes class balance...heavily armored Mandalorian mercenaries could rival Jedi at melee/tanking, while kortosis weave technology allows other classes to not be instantly slaughtered by Jedi in PvP. Assassin droids, Echani combat specialists, healing bots and Wookie Shaman...all the while the Jedi are less powerful than they otherwise would have been because the Exile's new Jedi Order is still growing, much of the knowledge hidden away by the Masters before KotOR2 is still missing, or whatever story you want to use for your explanation. On the Sith side the ravages of war resulted in the destruction of much Sith knowledge, but much yet waits to be found as a new Sith Order rises from the ashes of the destruction at the hands of Revan and the Exile.
Can a Star Wars MMO use the same faction+race+class model prevelant in fantasy RPGs and rival other popular MMOs in the long run? Probably not--the content doesn't lend itself to that kind of setup--but there are ways to make this work, with innovation and ingenious features like morality meters and more sophisticated class development, and interaction with NPCs that goes beyond quest-giving and quest-rewards that shape your character as much as any XP reward can, etc. etc. that play to the setting's strengths. So many cool things could be done if the developers were ambitious and patient enough--I think a KotOR MMO is very far from a guaranteed failure, though I cannot yet say how confident I might be in its success.