I like the art style.
Even though I generally dislike the trend of monochromatic, dark, gritty, muddy look of many of todays games that strive for "realism".
In Killzone 2 there still seems to be quite a lot of colour and contrast. For example the skintones of characters are a refreshing healthy red instead of the sickly gray that's in a game Resistance: Fall Of Man for example.
The explosions are bright and colourful, and generally there's a splash of contrasting colour every here and there in the game, even though the levels themselves are "gritty" and "dark".
More pleasing to the eye, and even good for the playability, that there's distinquishing colours in the game instead of varying shades of same colour.
MGS4, please add some colour too, I'd love you even more then
Colours are nice, as are flowers and hugs.