Stupid friendly AI, horde of never ending enemies and difficulty are the only down points. Still, a fascinating game!
The only downpoint is the difficulty, covering system and friendly AI. Now, that must be because i played the game on hard but from what ive read even normal mode is a pain in the ass to beat.
The enemies take way too many bullets to die, almost a full clip. This is kinda annoying because you die alot faster then them. Another annoying thing in this game, is that you can revive your troops (only the main characters) but they cant revive you. If you die once, game over. I lost count on how many times i revived a team mate only to die a second later right next to him.
Also in some situations, you try to revive them and run for cover as fast as you can. Problem is you must be exactly near them to use the medic tool (this doesn't happen in Multiplayer, you can actually be quite far from them). In these situations you will find yourself walking in circles while reviving your friend and be disorientated while trying to go back the way you came.
The friendly AI is also pretty stupid, as they tend to shoot at nothing and take ages to actually kill an enemy. They will often be in your way or stay in cover forever. Still, its very important you keep them alive, VERY important.
The enemy on the other hand, is very smart, way more smarter them your troops. If you arent fast enough, you will often find a Helghast right next to you, ready for the kill.
The covering system isn't that good. It feels there isnt a "safe spot". The enemies will always find a way to shoot at you, even trough the floor. It can be quite tricky, specially when you are reloading and with low health.
There are some points in the game where you dont know what you are supposed to do. Until you find whatever the goal is, the wave of enemies WILL NOT STOP. Nothing warns you here. Since most of the game evolve killing people, you assume you must kill, kill and kill, but no, in certain missions you must kill/destroy/reach a certain thing/spot and then the enemies will stop spawning. What bugs me is that nothing tells you what to do, you cant guess the wave in never going to cease.
An example of this is the "Secure the AA gun" mission, its actually the first one of this type. When you read the title, you assume you must defend it and kill all enemies near it. Well you are unaware that the wave will not stop until you actually kill the guy that is controlling that gun. I was stuck in this part till i read a walkthrough that actually said "you need to understand the game mechanics in these parts", obviously regarding the never ending enemies.
The weapons are alright i guess, but theres some useless ones like the Flamethrower. It takes forever to actually hit an enemy. He can be right in front of you and it will still miss. The Lightning Gun is the best one, to bad it only appears in two stages. All of them are nicely detailed.
Like ive said before, the game is kinda hard. Fighting armies of enemies can be frustrating sometimes. You have to calculate every shot and preserve grenades (this is extremely important). Another thing i found is that shooting "slowly" is way more effective then just wasting and entire clip on the enemy, since it becomes less accurate.
The best thing about this game is without a doubt the Multiplayer. Its just fun with 32 players and the variety of mods helps it. I dont know what reviewers talk about saying "people destroyed this MP", it doesn't make sense. At least when im playing, i always have fun with it. Sure, theres some campers, but most of them only play in the desert map, because its a giant one.
Killzone 2 is a great title and an excellent exclusive for the PS3.