I used my BitTorrent dumper to survey what was out there. What I learned is that BitTorrent isn't a file sharing network, it's a transport. Discovery and download initiation are still highly centralized. Every shared file is managed by a single tracker host that plays an active role helping find peers to download from. I fear most .torrent files are short-lived.
I did a survey from three sites of dubious legality: torrentfiles.com, animetorrents.com, and torrents.co.uk. In 870 .torrent files I found 32 different trackers. The top 4 trackers accounted for 75% of all the files. Not a lot of diversity. Of course, my sample is biased.
I was also interested in how folks use the BitTorrent metainfo. About 80% of all files use a piece length of 256k, followed in popularity by 512k and 1024k. I also found a bunch of unofficial tags: path.utf-8, creation date, comment, and md5sum.
I'd like to do a more formal survey with a wider sample; this torrent search engine claims 8200+ files.