The old image-close dialog (Cancel, Discard) was way superior to the new one
(Don't save, Cancel, Save), please bring back the old one. Reason for this is
simple, the new dialog causes the loss of data for the user.

Often one experiments with some image without the intend to save the result, but
now when one closes the image one gets the choice to save the results in the
close dialog, resulting quite often in excidently clicking on the save when one
really wanted to hit 'Don't save', since clicking the right most button is
really an almost automatic action, thus overriding a valuable image with
experimental junk. The old dialog on the other side made this excident
impossible since it only offered Cancel and Discard, so one never overwrote a
valueable image with junk, but instead when one wanted to save something one
just clicked Cancel and then saved the image as normal via the File->Save dialog
or simply pressed discard to discard the image. Losing an image by excidently
pressing 'Discard' was never an issue for me in the old dialog, since the normal
flow to save an image is not by closing the window with the image, but by going
via File->Save and only closing then. 

I have already lost one image due to the new dialog after just a few days of
using Gimp2.2, while I never lost any image with the old one in over five years.

Short Summary: Closing an image window should default to discarding the image,
not to saving it, which is an action completly oposite to the actual intent of
the user.