Can anyone enlighten me as to what \"Mark as compilation\" does?

It allows you to flag albums as compilation albums, if they\'re not detected as such. It also allows you to UNflag albums that have been detected as compilations, which you don\'t consider to be such.

To be honest though, amarok1.4 has always detected them properly for me, and I was under the impression it had been removed... although I notice it\'s back.

Generally, an album is defined as a compilation if it contains tracks by multiple artists, although some also consider \"Best of...\" albums as compilations too. I guess this last case would be one where you\'d need to manually toggle this.