I was looking for more info about this new feature, but could barelly find any. And since I've just got myself in the beta I figured I should post here some comparison screenshots with SSAO on and off.
Combined with the new ligting and shadows engine in MoP, SSAO (wikipedia link) really improves the visual atmosphere in WoW scenes. Even tho SSAO itself it's quite subtle unless you specifically look for it, overall it improves the image quality and depth.
Some comparison screenshots with SSAO off and then on - I recommend opening the pictures in different tabs of your browser so that you can switch through them for comparison purposes. Thumbnails are linked to the original size version (1920x1080) of the screenshots.
Scene 1- most notable differences are on the left side of the image and under the player's mount
Scene 2 - here it really makes a huge difference on the stairs and on the fence
Scene 3 - even in interior scenes the difference is visible
Scene 4 - it works even for objects in distance
Comparison screenshots taken from same spot in Org, between live and MoP engine.
The new mists/fog effects
Player reflections on water (not to be confused with shadows, even tho they look quite similar)
- currently on beta, SSAO only works with multisampling set to 1x (AA off)
- player reflections on water only show up on the 'good' Liquid Detail settings, once you set to 'ultra' they dissapear; also, the reflections are only visible from certain angles (camera close to horizontal plane) and are quite misplaced, they're not actually 'true' reflections.