Many good points made here regarding standard MMO stuff. I agree with most of it (although I have to admit I've never really played an MMO, so I speak from ignorance).
It is pretty likely to be just what people are saying, and that'll be that for me too. However . . .
it wouldn't have to be.
I can imagine a different kind of MMO, one which does include a DM-client. Why not? With instanced areas and mobs? The DM/Builder could also have access to both standard resources, to populate their areas for their group(s), along with the ability to use custom content for the same things. Core areas could be used at will by DM-led groups, so you could wander the rough streets and alleys of Dock Ward, Waterdeep, but when the DM decides a brawl is in order, an instanced area is loaded for the fight, with NPCs and all, but no PCs not in your DM's list. Other areas, designed and built by the DM/Builder, would be exclusively available to the DM and his/her group of players.
The official servers could run contests and such, discerning the best 'modules' (ie adventures) and content, and could even decide to add those quests to the stock game, making them available to other DMs. Maybe a deal on services could be made to entice the best DMs to do better and better. They could get free subscriptions to the DM client for their contributions, etc.
Now, this could be a pipe dream, sure. But isn't everything a dream, before it becomes a reality? This is an opportunity to leap up over the shoulders of the straining mob of MMO clones; to tower above like an iron fortress; to re-invent just exactly what an MMO is!
Whew, got a bit excited there, sorry.
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Of course, it'll most likely not be any of those things, but . . .
I will keep an eye on it, just in case.
JFK