Turn AO on and using the 0x00000003 (Fallout 3) setting has a pretty huge impact on how the game looks. Now that I've seen how it looks with it enabled it's a tweak I wouldn't run the game without.
Not a great shot, but this will give you an example of AO off and on:
Using 0x00000011 (Elder Scrolls 4: Oblivion)
Using 0x00000003 (Fallout 3)