It would be nice if compilations by various artists are not sorted 'by artist' but by the album e.i. the compilation itself (such an option is available in Amarok).

I think setting the album_artist tag (only possible in v1.1.x) to the album name will do just that.
By the way, I still need to use a default album_artist name when the compilation flag is on, maybe I could add an option to use the album name instead of "Various artists" or "Compilation".

Note that you can remove the artist tab and add instead a album_artist tab (with the small problem of a longer tab name  :(, suggestions welcome) by right clicking on the tabs.

About sorting songs by album_artist, I've just noticed that currently you can only sort using the "raw" album_artist, that is empty when no album_artist is defined. I'll add the "virtual" album_artist that is equal to the artist when no album_artist is defined. Suggestions for names to differentiate between the "raw" and "virtual" album_artist would be appreciated.