it does so much right, I had been meaning to make a topic, but never did.
1st off (and part of the reason im posting it here) is it blends so many genres together so well. at its core it is marines vs aliens, marines play like a fairly standard FPS. and they are loadout based, you can get various gear from weapons, and repair tools, to jetpacks and exoskeletons. while the aliens (i forget the term they use for it) are also played from 1st person, but play out more action oriented, they all have decent melee attacks, some more so than others, and they are more class based, with some traits (perks basically) that are shared among them and new abilities for each of them. and they range from super fast but weak, to tanky elephant rhino hybrids, to stuff in the middle like one that flies, or even a dedicated medic in many ways.
but on top of all that and what really makes it stand apart is that each side has a commander, they play the game not from the 1st person perspective, but like an RTS. they have an overhead view of the battlefield, and they manage resources and build things to assist the team, as well as research new powers or upgrades. but the commander also gives the game a very heavy focus on teamwork. it gives 1 person the leadership role, so there is generally always someone taking charge and organizing the team, and even games where the commander isnt doing that generally someone does step up and get direction to them.
its also got a better implementation of aliens running on walls and ceiling than I even thought would be possible. its just so much fun, and has plenty of tactical advantages to skipping between floor to wall and such.
EDIT: oops. didnt realize what day of the week it was, and that the free weekend was pretty much over. not really enough tim eto play it, but if you are interested in checking it out, might be able to snag it for 12.50 before it goes back up in price.