Tried WAR for a month; thought the PvP was freakin' awesome due to the tangible, open-world consequences that they built into the game; WoW simply does not compare. Even with leveling enabled in WoW, you still can't get the best gear without having to take part in scheduled, competition-style PvP matches with arbitrary or non-consequential ending points. That's changing, and PvP is not why I love WoW anyways, but WAR is just better at it hands down.
The problem with WAR is that I really don't care about Warhammer fantasy lore. It feels very, very derivative, even compared to WoW. You don't necessarily need a dynamite story to make a good game (not at all in many cases, in fact), but I feel like SF/Fantasy games need at least a token effort at universe creation, and WAR/Warhammer just doesn't give me that feeling, and its hard to play that long without it (for me).
A 40K MMO, OTOH, I might pay just to be in the beta. I want to be an Ultramarine Librarian, desperately fighting off wave after wave of genestealers as your own infected population turns against you. I want to be a World Eater Berzerker mowing down entire companies of Imperial Guard units with my bare effin' hands (and my trusty chain-axe). Be a Dark Eldar, and put an entire world to the sword just so you can toke up on the remains.
I kind of doubt that they would let us play Necrons, just because an individual Necron is a tad overpowered, and they would need to super-nerf them to keep it balanced, but still...