A couple things:
I think there should be more stats at the end of songs. How many of each color block there were and how many you got, number/size of combos, how many white/black blocks you got/missed. I dunno about most gamers, but when I play music/rhythm games like this I crave stats.
Also, it would be kind of nice to know how exactly someone who ran a song and only got Match 7 beats me when I have M11, clean finish, seeing red & butter ninja...
In general, I think the UI needs a lot of work. More media player style functionality would be a good start. Song search and browser needs to be a lot better. I'm sure I'm not the first person to suggest this.
Lastly, I'd love to have a giant log of all the scores I've ever posted too, similar to the My Songs list, only spanning the entire history of you playing. It wouldn't have to be much more than just a big text log file that the game writes and draws info from.