The problem with MMOTRPGs is, ironically, World of Warcraft.
Let me explain: World of Warcraft has a huge, monolithic share of a market, and by far the huge number of subscribers.
This can create an unrealistic idea of how well an MMORPG will sell. Hence, more investiment, hence more security wanted.
Because WoW is so clearly popular, the easiest way to 'insure' a game from the publishers point of view (or rather, what they think will secure it) is "WoW clone with".
Of course, in reality, people try it, normally have played on WoW before, don't see an imdient different/improvement, get bored, quit. Quiting rate gets high, word of mouth panic kills off the 'newbie flow', game subscriber base implodes.
What would really surprise me would be if someone created an MMO that worked in a similar way to the RP-MUDs of old. Second Life is like a modern Social MUD, so I have to wonder if an RP MMO would work. Of course, alot of players won't like a game that doesn't still to the WoW formula (bizzarely, they are turned off by familarity, AND by new things :| :\) but it could be interesting.