When comparing an item on the ground with your equipped item (using the Control key) the tooltip often overlaps the box showing the equipped one. This makes the feature very hard to use, and it is often easier instead to pick the item up first. In turn, this defeats the purpose of a quick comparison before picking an item up.

The tooltip overlap is dependent on where the character is positioned relative to the item, although I run into the overlap quite a bit during normal gameplay.

Screenshots illustrating this problem are here:

I had this problem running fullscreen at 1600x1200, but I reproduced it in windowed mode to capture it for documentation (the Diablo 3 built-in screenshot tool dropped the tooltip boxes from the resulting file.)