From Slate:

There may also be deeper differences. If the Internet were not a bookstore, or tubes, but rather a red-light district, YouTube would best be imagined as the hotel, and Napster, well, the pimp. YouTube, like a hotel, provides space for people to do things, legal or not. It's not doing anything illegal itself, but its visitors may be. But Napster, everyone more or less now admits, was cast as the pimp: It was mainly a means of getting illegal stuff. Right or wrong, we seem to accept the benign vision of YouTube as an entity which, unlike Napster, was basically born as a place to showcase stupid human tricks.