And one of them is ease of discovery. GitHub make’s it really easy to do that. Same with contribution.
As far as looks go? That’s more subjective.
But the discovery and contributions piece is easy to prove, just look at how much traction a project gets when it’s hosted there versus elsewhere. Even really large projects that has many contributors have moved there like WebKit
What I'm saying is, you could pick almost any popular Internet site, craft your requirements to describe it exactly, and write a blog post lamenting how there's no alternative.
It doesn't mean there's really no alternative (for you or anyone else) ... it just means you're announcing to the world that you are a big fan of that site, and you personally will never be happy with any other site no matter what.
At the end of the day, this seems to be the author's ultimate decision factor. None of the alternatives--open-source, self-hosted, or commercial--have the social gravitational pull GitHub does. Popularity begets popularity.