I thin Stubbs wasn't talking about the AO in your diffuse, but the AO in your world. Ie: intersecting assets. Without AO a lot of your props will look floaty. Also iyour scene has a ton of grey and kind of lifeless. I know its post apocolyptic but it still needs energy and personality. Your sidewalk is also a.bit blurry…
Ehh.. why are you using Mental Ray? Wouldn't viewport screenshots be more appropriate? On the scene itself: -looks a bit too happy for an apocalypse. -damage is inconsistent. everything is rusty, but no moss on the barricades. Hugeass cracks in the asphalt, but the fence is straight as an arrow. And so on. -you seem to…