WH40k could make a good MMO, but the main problem is, that it will have to have some heavy restrictions on who and what the players can be. And it will probably end up conflicting the official canon.
An excellent example of how restricted stuff can still work well is Dark Heresy (A tabletop RPG set in the WH40k world).
Oh, and like...someone mentioned: F*ck Consoles, we need some good PC titles.