Transmission not downloading
I'm using the Transmission which was shipped in 11.10. All the configurations in Ubuntu 11.10 are the same as they are in my Mac OS.
In Ubuntu, Transmission is not downloading at all: 0 peers at all times.
When booting into Mac OS, all seeds turned out to be fine (high speeds, lots of seeders).
In the Transmission menu go to Edit → Preferences → Network.
Check if your port (usually 51414 by default) is open, if not go forward by units (51415,51416..) until you find an opened one, this should work.
Be sure to disable your firewall for transmission (if you have set one).