As someone who's never actually really been into the Warhammer universe, i played the demo and i actually enjoyed it considerably - enough to actually sway my decision into purchasing it.
Although i've never read any novels or played any of the tabletop stuff before, i still know a thing or two about the universe, such as very general information on the various factions - Space Marines being genetically enhanced humans for example who treat their emperor like a God, the Tyrannids being a "we're going to slurp up every planet's resources for the lulz" copy of the zerg, the Tau being a "we just want to do stuff for the greater good", the Chaos Marines being the corrupted intergalactic troll varity of Space Marine, all that good stuff. (Unfortunately my knowledge of 40k lore ends at being able to recognise the races, in my eyes it just seems to be a bunch of races killing each other endlessly for no reason other than to kill each other)
I have seen a couple of warhammer figures before, when i was at high school a few people were into warhammer and i looked at their figures and gear out of curiosity. (Personally i thought all the little figures and designs and stuff looked pretty cool, but i just couldn't get into the whole tabletop thing, nor take it seriously enough) I always thought the whole warhammer thing would be far more awesome if it was on the big screen or as a video game, and now that is i feel able to get "into" it more. So when i see big-ass marines wielding chainswords and bolters that sound like a sub-woofer thumping into my ears i get the same sort of feeling i imagine actual fans of the universe do.
I think that people who don't know anything about warhammer will feel alienated. People who are complaining about the combat mechanics complain because they don't understand the nature of warhammer combat, particularly how space marines fight compared to what we've come to stereotype as "space marines" from gears of war and such. Space marines wear ten tonne of armour and wield powerful melee weapons. It would be incredibly impractical for them to take cover. Also, the way combat is designed is to make you combine ranged and melee. You're meant to blast approaching enemies, backpeddling as they approach you, and when they get close enough to swing their axe at you, you suddenly lunge at them with a move that stuns them and then rip their guts out with an execution from your chainsword, rather than stand there like a melon blasting away with your bolter and wondering why you're getting swamped, or mashing the melee button and wondering why it's not doing much damage. You're meant to melee one or two to restore your health, then run back to get some distance and drop one or two more with your bolter before you throw yourself back in there and execute a couple of stragglers.
Also to the guy who didn't understand the vengeance launcher: It's a remote detonated grenade launcher. You fire the grenade which sticks to something and then press the detonate button. (For the xbox 360 it's the RB button)