Nowaday, most boxes are behind some routers. Without upnp, bt is slow and DHT is practically useless.
There are already a few upnp library for linux. So please support using upnp to open port on the routers.
I agress upnp is important, but the problem is I don't have upnp capable router and I couldn't test the implementation.
There is software upnp internet gateway device for linux:
Description: Linux UPnP Internet Gateway Device
This is a daemon which emulates Microsoft's Internet Connection Service (ICS).
It implements the UPnP Internet Gateway Device specification (IGD) and allows
UPnP aware clients, such as MSN Messenger or games consoles, to work
properly from behind a NAT firewall.
Thanks for letting me know linux-igd. It looks promising.
Maybe you would like to try MiniUPnp (http://miniupnp.free.fr/).
Transmission uses this one.