Just some simple thoughts:
It had a good design.
Great search features.
A user moderated and created community.
Strict rules to keep it's "products" high quality.
Great branding. It was pegged as elite and hard to get into. Who wants to belong to a club that will have them?
Unique content - Hard to find vinyl rips, out of print records, stuff you couldn't get legally.
Did I miss anything?