Eric B and Rakim: seminal NYC hip hop; guessing classic hip hop is a blind spot in your data set.
Fleetwood Mac: mainstream classic AOR; Vetiver? An indie folk band? The Beach Boys are similar only as contemporaries --- also, not a very useful recommendation, since there are 3 different Beach Boys your recommendation could be keying on, two of them execrable.
Sepultura: insider-y mainstream metal; you did fine, I guess, but not better than the majors.
In two cases your recommendations are bad, and in two cases they're just not excellent. But if any online music retailer will do this for me, why would I use your service? To listen to some of the music online?
You should consider:
* Allowing people to upload their entire iTunes library files so you can analyze them and make targeted recommendations to stuff they already own.
* Allowing people to vote on the recommendations, or doing something "social" (gag) to let people adopt recommendations or biases from tastemakers who care about this stuff.
* Link to or repurpose content from Wikipedia to give more background for the artists you're displaying.
It still seems like this app is really just 10% of what Last.fm or Pandora already does.