There are always difficulties comparing one game directly to another game without a consideration of the different technical challenge in producing different games ...

Generally speaking, it is easier for a system to produce good looking indoor environments than outdoor environments because most indoor environments have a lot of flat surfaces which require far fewer polygons than the bumpy terrain and "organic" surfaces you tend to see in outdoor environments; and on top of that it is easier to organize an indoor environment into a well formed hierarchical structure (BSP-Tree, KD-Tree) to efficiently cull portions of the environment which are not being rendered.

Another consideration is how dynamic the environment is ... Games with highly dynamic and/or destructable environments generally require greater resources than more static environments because you can't pre-compute lighting, shadows, and a scene hierachy. In extreme cases, highly static environments can have large portions of their geometry pre-culled (basically deleted) because a character will never be in a position to see the other side of an object so the other side doesn't (really) need to exist.

Now, to aviod getting into the Killzone 2 vs. Gears of War 2 debate I will use a different example to demonstrate my point ...

Both FEAR and Crysis are very attractive PC first person shooters and you could argue that FEAR looks almost as good as Crysis which should be impressive because FEAR is a two year older game, and runs at full detail at high resolutions on a system which can barely play Crysis. The difference between the two games in technical challenge is almost as far as two games can be and remain in the same genre though and this is why Crysis requires so much more powerful hardware; Crysis has highly dynamic outdoor envionments while Fear has highly static indoor environments.

Gears 2 excels at having mostly flat surfaces that are covered head to toe in high res texture. Thats what creates the game's look. If Killzone 2 was in a forest environment you would see textures like that..........but the game is not. KZ2 is set in an industrial hard metal planet. There is no shrubbery, no trees. Only grid iron. If thats the visual direction of the game then so what?

Gears's textures are on point, no one is denying that. Yet so are Killzone 2's for the environment that they are in.

Gears 2 and Uncharted are much more comparable games because they have a lot of shrubbery and green going on in them with detailed architectural masses.