I know you guys have put a lot of work into this but for me, Compilations (Various Artists) work less well in 2.1 than they did in 2.0.2.

Here's the issue:

I've got a folder full of unsorted tracks (MP3), some of which belong to a 'Various Artists' compilation. So they are all tagged with the same album name but different artists names. When I scanned the folder, Amarok displayed them in the collection as a separate artist/album for each track. (I'm using Artist/Album sorting). So instead of:
Various Artists
  My Compilation
    Artist1 - Track1
    Artist1 - Track1
    etc
I got
Artist 1
  My Compilation
    Track 1
Artist 2
  My Compilation
    Track 2
etc

Now, 2.0.2 used to do this sometimes, but what always worked was selecting one of the tracks (eg Track 1) and choosing 'Show Under Various Artists'. This would move all the tracks on 'My Compilation' into the Various Artists section under a single entry for 'My Compilation'. What happens now is that it moves only that one track. So I did the same thing for all the others, and now I have (Num Tracks) copies of 'My Compilation' under Various Artists, and every copy has all the tracks. This is a definite regression over 2.0.2.

If anyone wants a look, I'll upload a selection of the tracks somewhere.

To add more mud to the waters, I've also got a bunch of tracks where the Artist tag is 'Various Artists', and Amarok was showing these under 'V', until I did 'Show Under Various Artists'; for these albums this option worked as expected.

As an update, it seems to detect the compilations OK if they're in separate folders. It's when I have one big folder full of stuff (including, in this case, multiple compilations) that it has trouble. I used 'Organise Files' on one of the multiple copies of My Compilation, to move the files into their own directory, and now it displays only one copy.