Just one of those games
The game is split about 50/50 between shooting and walking in a line. It's sort of strange how many rooms you walk through that are just empty. Sometimes (but not always) there's a little skull in the empty rooms. Those are audio logs. They relay extrapolations on what is already obvious. There are a lot of orcs, things are violent because of it, etc.
The shooting is fine. While you play as a big, slow guy like in Gears of War, there's no cover system. Instead the game focuses on having a high number of dumb enemies bum rush you, sort of like a really toned down Serious Sam. They do well with managing some level of variety within this setup by forcing you to prioritize the melee grunts coming at you with snipers on high in the distance taking pot shots. It's more or less enjoyable.
To deal with the melee guys you have your own melee attacks, usually either with a chainsaw sword or a magic axe. The game says there are combos, which is generous. You get a three hit string of attacks. At any point you can interrupt this string with a stun move. That's the entirety of the melee combo system. Basically, when enemies are close, you mash the button and hope they go away.
The stun move feeds into how health works. You have a Halo-style regenerating shield, but once that's depleted damage is fed into your health bar. Your health bar does not replenish until you perform an execution move on a stunned enemy. The result is that health restoration is extremely situational in a way not even the designers could necessarily predict. Furthermore, you're vulnerable to further damage during the execution animations. It's just not worth doing and melee combat feels very dis-incentivized overall. This is doubly true when 3/4 through the game the orc enemies are replaced with demons. The demonic enemies greatly prefer firearms and grenades over melee, all but forcing you to do the same.
Sorry if that's somehow a major spoiler. I don't know much about Warhammer 40k, but I found the story and setting in Space Marine difficult to care about. It's very basic. I always thought the 40k was more... morally ambivalent than this? The idea of religious zealots in huge armor flexing their dick muscles across the galaxy made me expect a grim take on Starship Troopers. Maybe not that far, but I wanted some kind of indication that the game knows these guys are assholes. But Space Marine plays things incredibly straight to the point that these fanatical supersoldiers are the unequivocal good guys. There's a weird story hook about how the main character can interact with 'The Warp.' They don't really explain what The Warp is, but it's apparently indicative that he's got chaos evil ghosts inside of him. Then they don't explain why the main character can interact with The Warp. He just does. The end. Okay??
Space Marine is okay. The best parts are when the chaos demons first appear and you get some good three-way fights between you, the orcs, and the demons. The melee combat is very undercooked despite being such a core feature. The AI is kind of dumb and likes to get stuck on the geometry. If you stand really far away and snipe at unaware enemies they'll just stand there and let you do it. Bullets don't actually come from your gun, they just hit the center of the screen so you can stand behind pieces of cover and shoot through them. The final boss is a Quick Time Event.
