Don't offer a free storage quota or limit how many projects you can host. There, problem solved. The people who leave will no longer be a drain on your servers and the people who stay will be paying for the maintenance of those servers.
Uhh... can't think of anything off the top of my head that I really need or want though... OH, wait, I know, how about Bespin integration?
I expect prices will rise soon as well. I'm not sure anyone has mentioned this yet, even though it seems inevitable. Still, I think GitHub is fantastic and I'm planning to increase my usage.