Quite often I download torrents on one site to seed to an existing torrent on another site because the data is identical.
This can be a bit awkward with Transmission because verify existing data doesn't tell you if files are missing or if the data is incorrect, so it would be nice to get some feedback on each file. I'd suggest just a message next to the "0% of x.xxMB" of file not found or data doesn't match
My other suggestion is that the folder name at the top of the new torrent window is selectable and copyable, because different trackers sometimes have different folder names so at the minute I have to type that out manually.
Also it appears that verify data only checks for the existence of a directory when you first open the torrent, if you then rename a folder to match the torrent and press verify then it won't find it, you have to press change then select to make it recheck the existence.
Keep up the good work, easily the best torrent program going.