Well, I haven't done any research on this. But I would definitely want a 40k mmo to be an MMOFPS, and not an MMORPG.
People will agree and disagree with me for this next line, but I think it would make a great Guild wars style game where the many story line is linear and the zones are instanced. That way you won't have to deal with seeing space marines run around grinding or anything like that.
The game should also be highly focused on multiplayer combat.
I still want to see an mmo that is RvRvRvRvRvR instead of the standard of horde vs alliance.
Also, Warhammer online was the worst mmo I have ever played bar none, including titles such as, "LotR online," "Age of Conan," "Star Trek online," "WoW," "Guild wars," and even "Final Fantasy 11"
1,000 witch hunters (trying to get rid of this)
1,000 SM (trying to get rid of this)
680 Tau (newest addition)