[hakim]

You're right – the intent is to demo the interaction when hovered and I did not spend any time designing the resting states. If this were to be used for something like a gallery thumbnail I think it's perfectly fine to not promote the hover preview at all. Users would discover it naturally as they mouse over to click on the thumbnail.

However if it's important the user discovers what's in the stack, this could be communicated visually like http://cl.ly/image/1c041l2e1S0O

[blake8086]

I think an obvious stack (perhaps even more obvious than depicted in your screenshot) would help. Another thing you could consider is having the stack give a little more affordance as the mouse pointer approaches. Perhaps it could expand a little to indicate "hey, there's a behavior over here!"