While Space Marine has had a good share of advertisement and promotion, I think its just unfortunate that it is coming out so close to all of these other AAA 'blockbuster' games. Deus Ex, Batman Arkham City, Gears of War 3, Battlefield 3, Modern Warfare 3, Uncharted 3, Halo Anniversary, Assassin's Creed Revelations, and so on (yes, there are more), it may get lost underneath the hype and excitement surrounding all of these other games. I don't know about everyone else, but I plan on getting at least four of these games, and its not going to be cheap.
But regardless, I do have fears that it will be accused of copying from other games, even though the 40k universe has existed longer than any of the aforementioned franchises (except the Batman universe, of course).
And to a degree it's true - I got a Gears of War vibe from the demo, and I'm familiar with 40k already. Some players who don't already know about the 40k universe might pick up Space Marine and just straight up decide it is a rip off, and ditch it then and there, without doing any more digging. Because face it, there are plenty of that kind of people out there.
Granted, I don't think the game will have any negative impacts on the hobby/tabletop game. People aren't going to stop playing 40k because of Space Marine. And like GML/BML said, there will be people who are intrigued by the universe and who check it out, but probably not as many as we'd hope. It still results in a positive impact though.
I'm hoping I'll be able to get Space Marine. But I see a lot of money leaving my wallet in the next few months.