Heavily edited version:
Playing the Killzone 2 beta is one of the best experiences I have ever had from a console or pc shooter. The graphics and animations are impressive, but they alone do not convey how finely the whole user experience is crafted. This game conveys the sight, sound, and feel of war to me so well that it feels like sensory deprivation that I cannot also smell the action.
I'm not going to pull punches in any way. I believe this game defines the first person experience on all platforms. It's a game that could only have been produced exclusively on a console, because only one of the big 3 could have given this game the time and resources necessary to mature like it has. It's really amazing. It wraps the weight of 100 pounds of body armour around you, and slaps another 20 pound rifle in your hands, and you really feel it. Rather than try and force a round peg through a square hole, Guerrilla Games has taken the tactile advantages of the console controller and built an excellent game around them. Two analogue sticks can't do a ballerina as accurately as a keyboard and mouse, and a keyboard and mouse can't give you the tactile feel of this game.
Tbh, this game gets far too much attention for its excellent visuals. People need to know that when you go from just watching trailers to actually playing the damn thing, you'll see that just as much time has been spent on the gameplay experience as the viewing experience. This is why, imo, the game will be very successful. It will not succeed due to a million different bullet points like lighting or animations, but because of the gameplay. The game is fair to noobs and experts alike, so it's really fun for everybody. There are also very real differences between the classes and ways to play online so however you want to fight, you're always going to feel like a part of the squad and a valuable member of the team. With the class system you can pretty much pick any role you want in the game so long as you've unlocked the prerequisites. You can be a soldier/scout who lets people know of concentrations of enemy troops, or you can be a medic/soldier and hang back to quickly get your friends back into the action and turn the tide in difficult engagements.
Supposedly the game has an excellent and long single-player game, and there's no reason to expect any less polish and design put into the single-player as the multiplayer after three to four years of development (and previewers back me up here).
All in all Killzone 2's both a top tier experience which defines what a FPS is about, and also an accessible one which will be loved by a wide variety of people for a wide variety of reasons. I think the best part about this game is the fact that so many people will be able to enjoy it, so those 3-4 years Guerilla Games spent working tirelessly, polishing this game will pay off.