A mix and mash of things already seen in other games, but still loads of fun.
Run here shoot these guys open this door, complete this objective, more bad guys to kill more doors to open. There isn't a while lot of originality here but they have managed to blend the fast paced shooting action with beautiful and sadistically satisfying melee encounters. The idea as role played in many of the cut scenes that the Ork horde relies on numbers in their strength as oppose to the Space Marines that rely on pure brute strength and lethality to dispatch their foes. And as a result, you have a perfect seamless blend of action packed shooting, and blood thirsty melee combat. Just don't expect anything too innovative and original.