This looks really great, but I have to take issue with your choice of typeface. While the description in the Google font database makes it out to be an award winning face for literature optimized for reading, its exaggerated serifs and uneven rhythm give it a kind of medieval / enlightened manuscript feel that just seems anachronistic and weird. For a general service that integrates with Github and shouldn't really be 'saying' anything with its design, I would recommend a more classic, rhythmic serif face along the lines of Minion, Caslon, Palatino, Baskerville, Hoefler Text, or even Georgia, which reads quite well at 14-16px.
Granted I'm a designer, I personally wouldn't use this service based on the typeface choice alone. It just clashes too much with the neutrality of everything else on my system and on websites I use commonly.
Sorry to hear that. I don't find the typeface to be as good as my current favorite workhorse serif (Elena, from process type, which I licensed for http://gazit.me )—but I don't find it to be objectionable or harmful to legibility either. Quite the opposite, I found it to be a serif with personality—less so than the commonly-used Skolar—and it reminded me of Elena (the shape of the adnate serifs, the slightly negative stress, the relatively-tall x-height, and the lowercase 'e' in particular).
I'll revisit type choices at some point, but right now the goal was to get something up, and as much as I love the faces you've specified, half I'm sick of and the other half aren't sufficiently ubiquitous.
For me your font-size is a notch too large, which amplifies the weaknesses of the typeface.
Personally I'd stick with Palatino[1] or Georgia for readability.
Mind you, this is perfectionist quibbles. Your font isn't bad. Just myself I'd probably hit the readability bookmarklet if I wanted to read a longer text on your site.
In my opinion, Elena is good choice. It's very good for reading, and that's the key. Yes, backend is mainly code-focused (github's gist), but people will most likely post not code, but regular articles. So there's hardly anything anachronistic about the typeface - github is just implementation detail.
As for Elena's character and style, it's there, but I always prefer some character over none - Freight Sans wins over Helvetica any time. Github uses Helvetica / Arial by default, so if someone wants default design, they can simply link to gist.
So once again - my arguments may not be the strongest, but I have a gut feeling that Elena is not wrong choice for such service, and if you like it, use it.
The one advantage something like Georgia has is that everyone has it local. Web fonts don't look great in all browsers and platforms - particularly on most Windows browsers, the anti-aliasing and hinting gets pretty heinous.
Granted I'm a designer, I personally wouldn't use this service based on the typeface choice alone. It just clashes too much with the neutrality of everything else on my system and on websites I use commonly.