Web 2.0 is all about participation and sharing. P2p is also exactly about participation and sharing.

Every user gets to become a  server and contribute to the community. Every user can simply share his files across everyone on the internet very easily.

P2p trackers are now capable to tracking usage data and then rating users. Leechers will be banned, seeders gets rewarded automatically, users who maintain higher ratios can download more files.

There are many communities that have evolved around p2p file sharing.  P2p combined with social networking is taking shape now. Allpeers, a firefox plugin is the best example.