A few independent musicians have noted that they make so little from Spotify (and the users don't get guilted into buying anything on iTunes) that it actually isn't worth it.

I think Spotify will slowly lose independent musicians and eventually lose market when they lose more artists (in fact, didn't a record label recently pull all tracks off Spotify?)

I can see a collapse approaching GroupOn proportions (let's face the inevitable there ;)) if things don't change soon, and for similar reasons (screwing over small business vs. screwing over independent musicians)