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?