Auke

Collapse ALL layers would be great.
A simple keyboard modifier when clicking would be nice.

It would be very nice if you could collapse all layers at once in the layers panel. Layers become very difficult to work with when you have many of them since selecting objects expands the parent layer, so you have to scroll back up and manually collapse them yourself.
Perhaps you could have it so that layers are only expanded for as long as the item within it is selected? That way, after deselecting an object, the layer expansion goes back to its previous state, which is ether expanded or collapsed. Am I making any sense?

I agree being able to collapse all layers and groups would be a useful feature. However I think it would get annoying if layers were popping opened and closed as you are selecting things. 

I usually disable the "Show Selection in Layers Panel" option in the User Interface section under Preferences when I don't want to deal with many many layers and groups in the layer panel. 
If I then want to find a selected object I can just right click and "Find in Layers Panel"