I'm sure everybody out there who use Bit Torrent is aware that being connectable is highly recommended for best speeds and the general well being of the "swarm". For the unfortunate few who aren't, connectability is where you open an incoming port on your computer thereby allowing other peers in the Bit Torrent swarm to be able to contact you. It is however only required for one of the parties (connector or connectee) to be connectable for a connection to be made. However, if your PC is not connectable you immediately lose the ability to download and upload from other unconnectable peers. While the number of unconnectable users varies from tracker to tracker (less for private trackers and more for public ones) and ISP to ISP, I found out first hand yesterday how bad unconnectability can be, on a friend's laptop.
On a well seeded torrent on a private tracker, with lots of connectable peers, we were averaging about 100KB/s. It didn't matter if we were connectable or not. His ISP is also notorious for traffic shaping and generally throttling p2p traffic. However, on a public torrent with about 10 peers, there was a (substantially) palpable difference in speeds. Speeds when connectable were about 80KB/s mark. However, when unconnectable, we couldn't clock anything greater than 8KB/s.
So, moral of the story: Be connectable; choose your ISP wisely.