Chris Brunner .com: Why You Shouldn't Run BitTorrent Over Tor
If you didn't already know, Tor is a distributed anonymity network that allows anyone to use the Internet to both browse the web and publish information without giving away his or her identity. It's a wonderful step in the direction of privacy and it serves an increasingly important role in today's world. As far as usability goes, Tor clearly has more potential than any anonymity network that I've ever seen. Tor could very easily be the most powerful tool that we as everyday people have to combat the gradual removal of our personal rights and freedom.
However, as of right now its most likely cause of death is not an organization or government, but rather its own users who in some cases, perhaps out of ignorance, take advantage of privacy the Tor network affords them by hiding behind it to steal software, movies, and music. I'm not going to sit here and claim that I haven't pirated my fair share of all of the above; that's not what this is about. Before you use BitTorrent on Tor, please stop and consider the effect this has on the Tor network.
And there's more….
Using TOR should also make you a bit more careful about what certs you accept: http://www.teamfurry.com/wordpress/2007/11/20/tor-exit-node-doing-mitm-attacks