It might be nice if my user page listing the fact that I like Sarge had the music itself, or a preview, a click away. But the important thing is the recommendation, not the trading, and to build a network uninfluenced by money, the distribution system has to be separable. So the software really should have a distributed database of people's recommendations, one that's not ownable by any one entity. Queries drawing on users with taste like yours should be able to come just from one community, or from the entire database, your choice. If I drop in Jonatha Brooke in February, and then 10,000 other people discover her in June, I need credit for being on the rising curve of the trend.
Time is crucial in measuring trends.
Corporate cool-chasers pay good money on research, every day, to find this stuff out. In a voluntary contributory system, everyone could have access to the same information, for free. Writing code that can be integrated into existing internet communities is key as well. Firefly failed because nobody went there. Everything from the database to the web interface was proprietary, and its information was only on one website.