In short, Space Marine straddles on the fence between good and excellent. It never crosses it, though. Let’s see how it could have.
Warhammer 40k is an interesting setting. I think many people would enjoy its uncompromising nature and big events. The problem is that 40k games tend to be quite heavy. The titular game is not for the faint-hearted, Dawn of Wars are complex (compared to mainstream games a la Gears of War) strategy games, Space Hulk and all its variants are mercilessly difficult and almost every supplemental game (Battlefleet Gothic, Dark Heresy) are leaning on the heavy side. So Space Marine, a light fighting game in the style of God of War and Gears of War, is the perfect game to introduce new people into the setting with minimal input.
Bearing this in mind, Space Marine should be a light game that focuses on presenting the 40k setting in the best possible light and being faithful to the canon. The last part is important, as fanservice is a major selling point of the game. Breaking established concepts incites nerd rage.
Regarding 40k setting, take a look at the intro video of Dawn of War:
This is what Warhammer 40k is supposed to be like: Uncompromising war, big guns, lots of explosions and zero subtlety. What the video does not show is the absolute religious zeal of the Space Marines to kill the enemies of Man and resist Heresy. In the Dawn of War, every unit (apart from the Necrons) charge into battle screaming their battle cries at the top of their lungs. Everything is loud and big. While not everyone believes Warhammer 40k is all out fury all the time, I do. And this is one part where Space Marine stumbles.
The main characters do not have any damn personality. Everyone is (ironically) the clichéd space marine (note the lack of capitals): Quiet and gruff, battling with grim efficiency and occasionally commenting on how many orks there is. Only the angry rookiee quotes the Codex Astartes, the guide of the Space Marines, but then Titus silences him with a few quiet words of wisdom. It would have been a welcome change of pace to play as a nutcase zealot who is the hero of the story on the account that everyone else is even crazier.
As I mentioned, being faithful to the miniature game is important to the fans. Relic has managed this exceedingly well in the visuals: Everything looks as it should, the architecture is appropriately full of skulls and the weapons sound, look and feel like their tabletop counterparts. I especially like the efficiency of the bolt weapons and the clicking sound they make when the clip is nearing its end. Although the prize for the best bolter goes to Fire Warrior, where the bolter feels like an automatic grenade launcher that it is. But the Space Marine organization is a mess. Captain Titus goes to war with two other Marines, a veteran Sergeant and Some Guy. I thought a Command Squad consisted of five units? The number of different Space Marines in the entire game is about fifteen, including cutscenes. Where are the tactical squads? Where are the assault marines? The devastators?
Rather than play as Titus the whole game, I would have put the player in the massive ceramite boots of different Space Marines. First a tactical squad battle brother, making the first planetfall. Then an Assault Marine Sergeant clearing the path for the Command Squad in the ruins of the city. Then a Devastator Marine, carrying huge weapons in an assault to an ork stronghold. And finally the Captain, who can use any of the weapons the player has now learned to use. Just a couple more character models and slight changes to the level structure and bingo: Fans approve. Every review score goes up a couple of points.
Speaking of level structure, it could have used a bit of polish. Just a bit, mind. The bread-and-butter action of the game is somewhat repetitive, though new weapons relieve the tedium: Huge room, shoot orks, move to another room, shoot orks. There are a couple of places where the action goes in another dimension, like the beginning. Anti-air fire prevents you from making planetfall with a thunderhawk (why not use drop pods?), so you leap from the sky using jump packs. An ork cruiser ruins this plan by being in the way, so you land on the upper deck, kill a boatload of orks and turn the cruiser’s cannons towards itself. Then you land with the crashing cruiser. Pure, dumb awesome. The vistas of the game are also exceedingly good looking: Almost every time you come out of a building, there is something huge and imposing in the horizon or something iconic flies past.
One of the most impressive features of Space Marine is how it handles health. Titus has regnerating shields stolen from Master Chief, plus a health bar. The health bar replenishes only by killing enemies with a special execution maneuver. So instead of hiding behind a concrete block and sucking your thumb, you heal by charging the enemies in melee. That is pure genious and fits the Space Marines like a glove.