I'm sorry but I have to Rant about this as not being enough for Co-Op, but then again I believe Rebellion obviously don't care nor have the experience to make a Co-Op dedicated AvP game.
Seriously, is this the best they could do for Co-Op???!!! Haven't they learned anything yet from the first PC AvP game? This is practically that same Skirmish BS endless wave after wave of Aliens only.
Why not allow players to team up and fight an Alien Queen (or Queens)
Or how bout teaming up against a bunch of computer controlled Predators?? Pred co-op would probably turn out boring, but why only Marines get to co-op against AI controlled enemies? Why not allow players to use Aliens and Co-Op against some really tough AI Marine Squads??
Have they even bothered to think about that for a second??
So now what? You can only team up as 4 Marines only??
Then that's not much of a difference compared to Aliens Colonial Marines, with the main exception that game is designed specifically for Co-Op, more so "Objective-based/Campaign+Plot based Co-Op"
Well then forget this then, I'd wait for Aliens Colonial Marines instead. Everyone else that disagrees, then enjoy mindlessly killing each other like any other Unreal Tournament and Quake 3 FPS clone.... since the MP modes concepts here are more or less the same, with some minor specific tweaks, but you get the picture.