Seems that Comcast was intentionally throttling downloads for torrents (streams of data from multiple peer-to-peer sources). Besides making downloads take artificially long to complete, this violates the new net neutrality laws.
From the article:
FCC holds nation's second largest broadband provider broke network neutrality principles when it throttled P2P traffic from BitTorrent. FCC rejects Comcast contention that network throttling is part of routine network management by broadband service providers.
eWeek posts: "Comcast Guilty of Net Neutrality Violations".