Yeah, the ones that force a share ratio. How inefficient that is! (For those who don’t know, you sometimes can’t even download until you upload some number of bytes, and you can’t upload because everyone else is also uploading.) Very non-dynamic. Plus, of course, the community’s policy doesn’t extend outside the community, and it still doesn’t reward good uploaders with more bandwidth on demand.
Then there’s the 2nd big problem of bittorrent. It doesn’t search your harddrive for things to upload. If you don’t have the .torrent loaded in your client, you can’t be sharing files even if you have them. Also, renaming files or move them around breaks the upload. This dramatically cuts the number of seeders on rarer/older torrents.