Alien Swarm.This game is a birds-eye co-op game that is qquite fun but has some issues.
1) Community - This game can be very fun, but nobody plays it. Working in teams of four is the only way to beat missions on tougher difficulties but the only way to have a team of four is to play with friends or on singleplayer (which sucls)
2) Class restrictions - While you can play as any class in the game 2 are required for most missions. The Tech, who unlocks doors and completes objectives is required for some missions and if they die, you lose. The Medic - THis is the main problem, because there is no good way to heal other than the medi-gun some player is froced to go medic. Not too bad sometimes but if no-one else your playing with has unlocked the medi-gun, it can be hell.
3) ITS TOO SHORT - A huge problem with this game is that it only has one campaign with 8 or so missions. These can be really fun but once completed there's not much to go back to.
Now that I'm finished with negatives, here are some of the postives
1) Classes - You can play as four classes, which are usaully fun with a good loadout. The Leader has a nice all-around skill, The Heavy just shoots the crap outta the aliens, the medic will save you from parasites and death and the tech unlocks doors, completes adjectives and is very useful. Oh and did I mention there are two characters of each class, for more variety :D
2) Parasites - Yes, I just said parasites are a positive thing for this game. Parasites are small aliens that latch onto your head and maul you unless you can be healed. While these can induced frustrating and quick deaths, they are a challenge that forces you to stay together with your medic.
3) Aliens - Of course they had to be in here! While not in to much variety, the types that there are make the game interesting (and sometimes a pain in the ♥♥♥!).
4) The weapons - You unlock weapons as you level up. Sentries can provide a quick escape from a quick death and the flamethrower is incredibly fun to use!
5) Levelling up - Levelling up gives you a sense of achievement as you shoot the crap out of aliens with that amazing new minigun you just unlocked
6) And finally loudout customization! - The game features 3 slots to change weapons, a primary weapon to kill things with. A secondary or piece of equipment. This can be more ammo, another weapon, a sentry etc. And finally the 3rd slot (forgot the name) which can carry passive armour, light attachments, welders (very useful) and other helpful things.
7)It's free too! (almost forgot.
In summary it's a fun game but it has a small community which can ruin it. But all-in-all you might as well try it out because its freee!