In Space Marine you play as a captain of an Ultramarine squad and you charge through the battlefields of an Imperial Forge World called Graia, fighting various orks and later on chaos (not really a spoiler, as it's obvious because of the multiplayer). The campaign is a moderate to long campaign, but Space Marine doesn't really shine in general, but in specifics.
In Space Marine you have the usual assortment of weapons taken from the Warhammer universe, a sniper rifle, a bolter (your main gun), and upto heavier weapons like the Heavy Bolter, the Plasma Cannon and the Lascannon. They are all well and good and nothing really blows your mind there (although a headshot will still blow the enemies). Where the game really comes into its own is the melee, and it kinda feels like you need to charge headlong into every fight. For starters you cannot take cover, something that has been force fed into our gamer psyche as a necessity in recent shooters.
Secondly you do not regenerate health as you would in COD or Gears of War, and there are no health packs as in Unreal Tournament and Left 4 Dead. To get your health back you have to execute enemies, generally standing exposed in a group of them while you're doing this. What this equates to is two styles of play. The first is standing behind a wall, nipping out to shoot enemies and then hiding again, which makes you feel like a total woss, or going insane in the melee, hacking your enemies into as many pieces as possible, which is fun but you dance a very fine line between massacring them and death.
Fury is another mechanic of the combat. Basically getting kills' fills up the meter you can see in the above video on the bottom left. When this is full you can active your Fury, which basically makes all your attacks more powerful and refills your health. Later on you also unlock Fury to help with your shooting, making everything slow motion firing.
Another mechanic they have in the game is something that you will have seen before in Halo, Jump packs. Obviously they were in the Warhammer universe first but Halo did it game wise first. With the Jump pack that you'll get automatically as certain campaign sequences you can leap into the air and crash down, smiting your foes. It's not that show stopping but it break up the campaign and is fairly enjoyable :)
The multiplayer on Space Marine is fairly straightforward, like the rest of the game really. There are two game types, a team deathmatch and a take and hold gametype. In these matches you are either a space marine or a chaos space marine, and there is a moderate degree of customisation. As you complete weapon challenges you unlock different armour pieces (cosmetic only) and various perks for the weapons. You can pick the colour of your armour as well, so you do get to make yourself look like your own miniatures if you own any, which is something I'll come back to below. The free DLC that came out in October added the Exterminatus game type, a 4 player "survive the wave" gametype that is becoming more popular since Horde for Gears of war and then a little later Firefight for Halo.
Space Marine is a fairly clear cut game. Its simple and straight to the point. The melee is fun, the multiplayer is enjoyable. Also, the nostalgia value of playing as marines, killing orks and chaos and being chaos in multiplayer as well, coupled with the customization of the multiplayer takes this game to another level for Warhammer 40,000 players. A good game, and worth trying, but probably not the game you'll be spending the foreseeable future playing.