Apple TV 3 on top of AirPort Express 2nd Gen.

Heat will probably be the biggest factor. I had a switch sitting on top of my AEBS briefly and it only took two days before the heat between them affected network performance. Once I set them side by side and they cooled down, things worked as there should have.

Having them on top of each other will affect the wifi, but it will probably be negligible and you won't notice it.

I would recommend not stacking them unless you have plenty of separation and/or active cooling such as a fan blowing on them.

 

I don't see why not. I'd get a powered USB hub just to be safe in case you ever need to run devices that need it.

 

I always wondered how those Apple boxes dissipate heat, I mean it looks plastic all around and the bottom has a felt covering.

 

They passively radiate heat from the enclosure.

 

That would be my guess. That must be some high tech plastic.