Don't freak out guys. This game is overrated, but it's still a great game.
If you want to hear the good about this game, read any of the surrouding reviews...I agree with them. I will simply point out the game's many flaws, but keep in mind that I think this is a very good game, and these are really the things that dissappointed me.
Gameplay
The gameplay in Killzone 2 is solid, at least in single player. The difficulty is fairly well tuned, though it may be a bit on the easy side. The weapons feel pretty good for the most part, though the Rocket and the ISA Rifle are pretty weak (The Helghast Gun is F***ing awesome!).
Graphics
Stunning...but flawed. The art design is beautiful, but the graphics really aren't that much better than Gears of War 2...in fact, I would go so far as to say that the graphics aren't better at all. What makes the difference is the geometry. Everything just looks real in Killzone 2, but when you see something that doesn't fit, it REALLY sticks out. There are bad textures all over the place and the physics engine just flat out sucks (ex. The rocket launcher. Seriously, fire it at a Helghast, I dare you. The poor bugger will move about 2 inches, as opposed to flying through the air a la the first Halo. Whoops!) Killzone 2 is a painting. It looks great when still, but if you try to move it, its flaws become more obvious. It is stunning to be sure, but not all that it is hyped up to be.
Sound
AWFUL. While the ambient effects are awesome, the weapons are pathetic, with the lone exception being the mind bogglingly awesome Helghast assault rifle. The ISA rifle sounds like a pea shooter, and the Rocket has the weakest little "bam" I have ever heard. This game will stun you at first, but the sound effects won't hold your interest for long.
Multiplayer
Solid, but there are balance issues. The Helghast assault rifle is overpowered compared to its ISA counterpart, and quite frankly, this game is going to suffer from the same problem that plagued Syphon Filter on the PSP: the starting weapons are no match for the high ranked weapons. Sorry guys, but newcomers are going to be few and far between, because noobs are going to be MEAT in this game.
Killzone 2 is an achievement for its developer, but it isn't anything revolutionary. It does what it does well, but what it does has been done one hundred times before, and it has been done better. Definitely not a Halo killer, though a wonderful improvement over the first Killzone.