I once used a photo management tool in windows (FastStone) which had an excellent option to compare several pictures. In the browsing view you could select multiple photos (ctrl-click) and then choose "compare images" in the context menu. The chosen photos where then shown side by side in fullscreen-mode, which made it really easy to compare which image to delete and which to keep. When zooming in on a detail, all images would zoom along, so it was really easy to check which image was particularly sharp or which image had a certain spot better illuminated.

I often take many shots of the same subjects, so when importing photos into f-spot it is usually one of my first tasks to sort them through and keep only the very best of the duplicates. The comparison-view described above would greatly improve this workflow, especially if it would also allow to easily delete the unwanted pictures.

By the way: I would love a keyboard-shortcut for rotating pictures (in IrfanView it is simpyl "r" or "l" depending on the direction, and a keyboard-shortcut for deleting pictures from the disk; as it is, the delete button only offers to remove from the catalogue. In 9 out of 10 cases I want to get rid of the file entirely.