The best thing about Napster was getting almost any music you could think of in a moment, for free.
The second best thing, a close second, was surfing other peoples' collections. For a few months in 2001, almost every week, I discovered new bands and unknown albums and singles of bands I knew already. I would browse the collections of people who were sharing stuff I liked, and sometimes just download everything -- everything I didn't already have. Since I was on a T1, that could take just a few seconds in some cases. I would have a week of new music to discover, from the collections of people around the country and possibly the world, in an instant.
I wish there had been some way to pay for Napster rather than having it shut down. Both iTunes and the new-style (free, illegal, Torrent-based) file-sharing services pale in comparison, both in user experience and in sense of community (or lack thereof) -- Napster had community, even though I never once made any contact with another user I didn't know, except by downloading and sharing music with them. I'll always be a Napster kid.