GitHub isn't polished. It just has engagement mechanisms like twitter does, that keep people coming back. When people say how much they like GitHub they're usually talking about the community or git, both of which GitHub takes way too much credit for.
> And there are other DVCSs. As it is, the main hosts for bzr and hg are clearly less slick and less popular than Github. But had Git not been there, it's quite plausible that someone would have built an equivalent site around another technology.
There were a lot of people moving to git from hg when GitHub came out. I think it was clear there was demand, and if not for GitHub other services would have sprung up to fill in the gap.