It would be all about the mechanics in my opinion. There is really no healing class in 40k... so that means the game would be a potion popper like Diablo and in terms of MMO's... RF Online. That in my head doesn't make an interesting game.
I see this game being souly about large battle PvP and very little PvE elements.
Items and Gear would most likely be based off the armies existing codexs... bolters for SM, shukien catapults for eldar, etc.
The more I think about it... the more I think this game will be exactly like RF Online except with a better story line. But really, all it is is large PvP combat and unbalanced classes. Also a espically boring grind session to get to be powerful enough to do anything.