This game will stride through the competition and cut them to pieces. You must see/play/buy this game. 
No matter which words nor how many I use, you will not know how cool this game is until you see it for yourself. However, I will attempt to do it some justice in the following paragraphs.
The Warhammer 40K franchise has seen some fairly awful games. More recently it’s also seen some quite good ones. But none of those compare to the greatness that will be Space Marine. Just as the Space Marines are towering seven-foot monsters armored like a tank and sweep enemies aside with but a wave of their arm, or chain blade, so to will this game do the same to its rivals.
You take on the role of Captain Titus of the Ultra Marines (who is not bald), in one of the best third-person shooters I’ve seen lately and was a complete surprise. I was worried that, as the first real shooter in the series, they might screw it up. They have not.
You will play through several types of missions that emphasize what it means to be a Space Marine. What is
that you ask? It’s not being afraid, not cowering behind a cement block and never, ever backing down until you or the enemy is dead. In this game, the enemy is Orcs, and you will go to town on them with your Bolter, your Chain Blade and several huge set piece weapons. There are 15 weapons in all including some exotics, like the Heavy Bolter, that are uber-powerful and break up the run-and-gun of the game.
The story goes that some Orcs crashed on a Forgeworld, a planet used solely for the production of war machinery for the Space Marines, and are tearing the place up and making it their own. So you and your squad are sent in to clean things up.