How to get old Dock Exposé on Mountain Lion?
Thankfully, this feature isn't completely gone. It has just, for some reason, been whisked away into the depths of less-than-obvious multi-touch gestures in OS X Lion and newer. You now have to double-tap an app icon on the Dock with two fingers to activate Dock Exposé.

I'm not sure if this gesture ever existed on OS X Snow Leopard.