Everything looks like it has been made from the same material. You have dirt, brick, tile, plastic, metal and glass... and it all looks like it was made from the same materials, a semi glossy clay. This is evident in not just Halo 3, but other games as well.
Here is Haze:


Here is uncharted:

His shirt and the arches could both be rock. It's just one is lighter and shaped like a shirt.
Halo 3

The rock in the background and the helmets might as well be color palette switches.
I could put more Halo 3 images as well, but most are pretty familiar with what that looks like. The most offending is one halo 3 water image where a player is stepping in it and it doesn't look like he's stepping in actual water but some soft metallic liquid. Everything seems to be from the same material, and I am really noticing this within this gen of gaming. Recolor grass and its ground, recolor metal and its grass. Dirt doesn't seem to be a different material from metal. It's just colored differently. In 5 years everything will look like glossy clay to us. They need to figure out, or hire someone who can figure out how to make the individual elements seem different.