Why does this game deserve such a high score? Well its free, polished, and most importantly a boat load of fun.
Story:
This sadly is the weakest part of the game. The story is told through texts screens that you probably won't bother with. It has something to do with a alien outbreak on a colony somewhere in space and a team of marines need to stop the outbreak. Its really not that engaging.
Gameplay:
The gameplay is basically a over the head shooter with swarms of enemies rushing towards you. You need a squad of four people to simply get from point A to point B with some obstacles in the way like locked doors, alien eggs, debris etc etc that you need to clear to get to your destination. The thing is though you don't have regenerative health, limited ammo, waves of aliens of all different shapes and sizes rushing at you, and you need a technician to open a locked door what do you do?
Well this is where the fun begins before every mission you need to pick your squad/yell at your friends or teammates what their roles should be. There is a total of 4 classes each with 2 characters in each. For example there are 2 medics one is better at combat another can heal you quicker and you need to pick accordingly. The four classes are technician, medic, officer, and special weapons. Each class has a specific skill or talent. The special weapons character is the only one who can wield a massive chain gun weapon. The technician can wield doors (makes it harder for enemies to breach) faster and is the only one capable to unlock certain doors. The medic...you guessed it is the only one who can heal. The officer really has no point in the game they have a MASSIVE shotgun that does amazing damage, but nothing else.
Now what makes this game work is the roles you have to accept the technician has to be protected or at least the best player of the group. There are certain doors that have to be opened to finish the mission and if the technician dies before he can open them the whole mission has to restart. Another is someone has to be the medic especially in the later parts when there is something called infestation. Basically if its not treated you will die from it. There also needs to be someone carrying a ammo box. The ammo is thankfully universal meaning anyone can carry it. Let me tell you ammo in the game is very hard to come by. Ammo clips act like real ammo clips what I mean by that is in normal FPS/shooter games when you reload you just add more bullets.In this game you reload you throw your clip on the floor and put a new one on. So that clip you threw on the floor had 40 bullets left in it, then kiss those 40 bullets goodbye.
There is also side items in the game as well that you and your team have to discuss about. It has decent variety from the door wielder to 20% less damage or a personal health kit these items are just as vital as the weapons you choose.
Speaking about weapons and items there is a unlock system in place and let me tell you it is very addictive. Like COD you need to level up to use certain items or weapons and they range from amazing to meh. Certain classes get bigger and better weapons, while items you unlocked range from useful to useless.
Another thing about the game that really helped in its score was it is extremely polished, gave everyone the source codes, achievements, and is supported. Right now when I started steam there were fixing problems people had with the game. I personally found very little problems and thought a game this well polished couldn't be free, but I was glad I was wrong. I also can't wait for the user created mods that the community will use from the source code.
Yet not everything can be perfect can it? the main problem I had with the game is it is very short. The game only has 1 campaign which is only 7 stages. Some stages (mostly in the beginning) are only 5 minutes long. Also since this is from the makers of Left 4 Dead you will need a good team or friends to help join you or the game will not be as much fun. Also like I said before the officer class is really a waste no one ever plays as them.
Graphics:
I have to say I am impressed they are very nice. The aliens, marines, gun fire, explosions, facilities all look great. They even added a physics system in the game!
Audio:
The audio is pretty good as well. There was never a moment where I hated it or laughed at the dialogue or whatever.
Pro's
Its free, user created content should come soon, its a ton of fun, very well polished, very frantic, valve is supporting it, some epic moments, interesting squad mechanics, addictive leveling up system, and finally once again its free and a boat load of fun.
Con's
The game is short, you need competent teammates, officer class is a joke, some unlocks aren't good, infestation is annoying.
Overall:
Overall the game is really fun and you will play it for hours on end. My only wish is as of this time of writing this review there is no announcement of future levels for this game. I hope valve makes a few more campaigns for this game because I will happily pay for them.(if they decide to charge for them)