Well, apart from the experimental weapons,(the vengence launcher)
Its pretty close to the way most 40k fanatics would imagine.
Cant wait to play the full game
Yeah, the weapons are the main thing that deviate from the canon. For example, you manage to find a Power Axe, a Plasma Pistol, a Plasma Gun, and an Iron Halo. All things a Space Marine captain would have access to - a Founding chapter's captain no less, but just finding them on a world under siege is strange. But, I'm a 40k nut and I don't mind it. They're there for gameplay, and I think pretty much everyone will agree that having actual options in both your melee and ranged weaponry makes it even more fun.
I won't venture into spoiler territory, but a key plot element seems like it'd break the canon, but Relic tends to leave things vague in the first installment of anything. I imagine that if there's a sequel, it will explain more/continue the story and flesh things out a bit more, as Space Marine has gone through a few overhauls.
I've heard that people sya that this game is very faithful to the Warhammer IP. To be honest I have no Idea what IP means. I'm assuming that what is meant by this is that this game is very faithful to the source material associated with the original tabletop game.
[Edited by Dlenny2259, 9/8/2011 2:31:48 PM]
It's not hard, really.
In this context IP means Intellectual Property; it's referring to the source material provided by Games Workshop.
The fact that the game is true to the source material is a Good Thing; the Warhammer 40K universe is popular enough that anything less would be a huge affront to the playerbase. Staying true like that, BTW, is one reason why the Dawn of War franchise is so popular - people expect certain things from any WH40K game, and Dawn of War delivered, for the most part.