I read “Interactive Digital Photomontage” paper today, and I think that it is freaking amazing:
We describe an interactive, computer-assisted framework for combining parts of a set of photographs into a single composite picture, a process we call "digital photomontage."
As I can see, there exists an implementation in C++, released under GNU GPL, I don’t have any clues how much effort it would take to have this feature enabled in F-spot but I believe it can be major reason for one to switch to it.
Also, I do like how Aperture can group stack frames, and unfold them only on request. Support for linked RAW files (stored on external removable media) is also very nice, but as I am not professional photographer, that’s not important to me.
On the other hand, it would be nice to have some support for importing photos from iPhoto (and Aperture). I read once about some Perl script that can do that, but I can’t find it right now.