I asked this question a while ago. I learned.
Killzone 2, simply, is an awesome game. It's not going to save the PS3, that's something 360 fanboys keep claiming the PS3 fans say, and I have never seen a PS3 fan saying Killzone 2 will save anything, but it will be extremely good.
It has better graphics than anything on 360. Boom.
It's an exclusive FPS that's excellent, and "AAA" which in today's terms means "90+ on metacritic". The PS3 now has one of these.
The cover system appears to work extremely well.
We have gotten nothing but unanimous approval from reviewers and beta testers alike.
But finally, and this is perhaps the most important, it broke out of a mold. The mold that all hyped PS3 exclusives will flop. MGS4 didn't flop, people called that a one off. LBP didn't flop, but of course it did, because it hasn't sold 10 million units yet. Now a third game hasn't flopped.
Killzone 2 is a lot harder to discount than MGS4 or LBP. First, all the other games "aren't AAA" which like I said has become "less than 90 on metacritic", even if some of them are 88 or 89, so they don't count.
MGS4 is clearly a movie, because it has cutscenes, despite the good 15 hours of gameplay and full online segment. It also has a bad story (apparently)
LBP, oh god. It's kiddy, casual, 2D, and hell, not even a game, but a level creation toolset. That one was too easy, because a lot of Sony fans thought that...before playing it and seeing how awesome it was.
Killzone 2, on the other hand, is difficult. It's an FPS, hardcore in every sense of the word, with incredible graphics, a seemingly great story, it's fast-paced, action packed, and it's been compared to Gears and Halo, and in many cases thought to be better.
When a game is compared to Gears and Halo, you can't discount it without discounting Gears and Halo.
So I suppose now, in the eyes of the 360 fanboys, the PS3 has ONE great exclusive. Just one.