Github is a big, big deal. I don't think it is a big deal just because it is underpinned by Git (and thus "better" - by sheer popularity - than Mercurial/DARCs etc).
I think it is a big deal because the developers of Github realized that developers are a social network. Github have done the social aspects of coding so much better than the other offerings from Google, SourceForge etc. Social computing seems to gravitate towards a small number of thematic nodes: family - facebook, business - linkedin, musicians - bandcamp. In the same vein : Coders - Github.
Git concepts like pull requests etc. certainly help to enable linkages between developers but it is github that gives all those dev-social interconnections a place to hang out in cyberspace.