Biggest problem with MMORPGs is the exact opposite of what you just named. Biggest problem are immature, brain-dead teenage power gamers who don't role play, but rather ruin the game everybody with their experience grinding and t3h ph4t l00t hunting. When you try to reason with them and explain that statements like "LOL fagg0t n00b" are OOC, you are met with a wall of derisive insults and a painful realizeation that morons don't even know what "OOC" means. No matter how much effort a developer puts into creating an RP-friendly environment, the servers will always become overwhelmed with level 60 kids with most powerful gear (consisting usually of three or four items that are worth something) to the point where frustrated roleplayers will just give up and quit playing the game.
The only example where MMORPGs work are roleplaying shards, but playing with 20 or 30 people kills the whole point of mass online gaming, since you can have a similar RP environment with Neverwinter Nights, minus the cost, with better graphics, a functional DM client and the ability to easily generate your own content.
MMORPGs... bah.
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