I was totally sold with github as comments, but then I saw you need to leave the page to comment. That's really unfortunate! If we could get a minimalist, not tracking widget that would be ideal.
I think expecting your users to have a GitHub account to leave comments is a terrible idea. Normal non-developers do not have, do not want, and should probably not be enticed to, make a GitHub account for purposes that have nothing to do with GitHub. Not to mention that unless GitHub themselves state that this is perfectly acceptable, they could take steps at any time to counter/block such usage.