It could be any number of things - internet connection, port forwarding, seeds, HD space, etc.
Concerning uTorrent specifically, I experienced one known bug. When you download a torrent, you have to download the entire thing and not selected files. If you attempt to download, say, 3 out of 4 files, you will get an error message stating that you're running low on disk space.
Other than that bug, uTorrent has been working great. I set a new record on my Mac earlier today... 11.3MB/s