[afro88]

Spotify does that already too - right click a song and "Start Song Radio". This will automatically pick songs that are similar.

I know it seems like this is what you want, but for me I've never been satisfied with this simplistic approach to discovery. Taste is very complex, and for me saying to someone "I really love xylophone" and them recommending me a lounge jazz tune with xylophone will always fall flat. What most people don't realise is why they love certain instruments or tempos or genres etc. It's usually down to how they were introduced to these, which defines their archetypal music using these instruments for example. It's all contextual.

My context for liking xylophone goes something like this "I love xylophone, because in the late 90s I listened to a lot of Beastie Boys 'Hello Nasty' which has some really nice pieces of music with some great xylophone in them. Then hearing some Mulatu Astatke recently got me interested in xyolophone and ethio jazz in general, combined with a really cute reaction my girlfriend had to one of his songs."

The best recommendation engine I've seen is Spotify Weekly. It's really very good, but make sure you get in there and save a lot of artists and albums you like first