Disclaimer: Not sure what's new, and what is not.
-Set months after Aliens the movie, on the planet LV-426.
-Some sort of
-It will fill in some story gaps such as what happened to Hudson and Vasquez. Also how the eggs ended up on the ship on Aliens 3.
-One new enemy is a bull-headed enemy. Think a regular sized soldier alien with a Queen's huge head. It uses it as a battering ram, and a bullet shield. You'll have to take it out with non-conventional means, such as from behind.
-Enemies like the bullhead will encourage the co-op play. "Take a Stand" moments will as well. Those are basically parts where the game throws massive amounts of aliens at you. The key is to reinforce your defenses (including turrets of course) and survive as a squad.