I just compiled Aqualung (my first compile

)
It isn't mature imho but it looks promising.
Their Relative Volume Adjustment System irritates me, the prescription quite fits replay gain
but I'm not sure if it's the same.
They have a separate setting for replaygain tags in .ogg and .flac, so I guess RVA is something else.
The 'music store' media library is a bit inconvenient too, but you don't need to use it.
I'm not sure about unicode, it looks like the playlist can handle them while the music store ignores those tracks. I might be wrong on this.
Well, it has some other features too but I didn't try everything yet,
all in all it looks like they are focusing on the imho more important, audio related features.
I definitely keep an eye on Aqualung
Edit: I tried version 0.9beta6 and will try the SVN version as soon as I manage to compile it ^^;
Edit2: There are quite some improvements in the SVN releases (tested R555), building the music store is now possible in different ways, by different file structures and by tags. Tag editing may still be buggy due libtag bugs but I haven't tried it yet. The german translation is still funny, I may offer them some help on this. Anyway, it's very slim compared to amarok and also very usable.