Basically, Space Marine fixes all the things that has gone wrong with the Third-Person Shooter genre. It removes the infinite amounts of chest-high walls and all cover mechanics, while still actually having levels where there's places you can go and take cover if you want/need. There's some regenerative armor on you, but your main health bar does not regenerate. To heal yourself? No taking cover or scrounging for health packs or whatever... you wade into combat and execute an enemy with vigor and zeal.
This game gives you good shooter mechanics while keeping good melee mechanics as well. You truly do feel like a Space Marine as you use your Bolter, Plasma Gun, Lasgun, Meltagun or whatever and then seamlessly wade into melee combat with your chainsword, power axe, two-handed power hammer or whatever. You don't experience war as a Space Marine. You ARE war.
You'll fight hordes of orcs (choppas, shootas, rocket boys and everything), vicious demons, psykers, orc warboss, nobs and so on and so forth. You'll supercharge a Titan and go toe to toe with the forces of chaos. This Forgeworld stands or falls with you.