Well, right now, if you're using the GIMP and don't know what the actual button is for what you want to do, chances are that you'll have to use a tooltip. This takes time, and while it's a very short amount of time, it can add up to be perhaps more than it really should.
Reactiveness: It's important go a great majority of users. Tooltips, while informative, are not reactive in nature.
What's odd about the GIMP? Tooltips are used almost exclusively for descriptions, but the GIMP places prospective labels to buttons in their tooltips as well. Perhaps relegating tooltips to take on this additional task isn't the best choice.
What would I envision in its stead?
Okay, say you've got your mouse hovered over a button on the GIMP toolbar. There is an area of text above the mess of buttons that will tell you the prospective label of that button. When no button is being hovered over, it defaults to the prospective label of the selected tool.
I have seen this kind of revealing used primarily in games and gaming systems. Take, for example, this screenshot from Majora's Mask:
Not one item in that menu throughout the game will have a label beside it, yet there's still a place you can expect the Deku Nut label to show up when you're navigating through the menu.