Did you ever need to copy a layer mask from one layer to another? In Photoshop this is a very easy process, which involves only the mouse and the Ctrl key, but in GIMP things aren't that straightforward (but not much complicated either).

This started to become annoying. Although GIMP has some very nice features and can be considered a real candidate to Photoshop, some of them can be quite hard and counter-intuitive to use. One of them would be the ability to paste a layer inside a layer mask - or in other words use another layer as a layer mask.