I agree with you that the technology is secondary, but one of the biggest things that I like about Github is that navigating through source files is always very fast.

I hate having to go to a source control site where it's not immediately obvious where to get to the source, or even worse, where it takes multiple seconds for a page to load.

With Github I can flip through all the source files to see what I'm looking for without having to bring the repo down locally. The same thing is also very useful to me when I want to remember how I did something in some project but don't have all my source with me. If navigating between files was slow or cumbersome, I don't think they'd be nearly as successful.