We've talked a bit about our typical traffic in the past (https://github.com/blog/1992-eight-lessons-learned-hacking-o...), but suffice to say, we host some highly trafficked sites like the Bootstrap documentation.
If your blog isn't on that scale, and conforms to our terms of service (https://help.github.com/articles/github-terms-of-service/#g-...), we generally don't enforce hard usage quotas, but if you're concerned, feel free to reach out to support@github.com any time.
If these can be added to the free version, cool! :)
Full SSL support, integrated build automation (not just for Jekyll), redirects, built-in performance optimizations, rewrite rules, proxying, fine grained HTTP header control, integration with prerender services, etc, etc...
We can also do custom plans with true DDoS mitigation.
Why not use them to present customer projects (in private repos) to your customers? As long the pages are public this is pretty often a no-go.
Github pages is so cheap and easy it is the disruptive technology that is eating the lunch of stand alone hosting. Users don't think about servers, deployment or other things, they simply are pushing a branch and poof it is on the web.
Do you have a wishlist of features to add to GitHub pages? Maybe allowing minimal sandboxed server side computation with a max runtime of say 1ms, setting headers, redirects or other stuff? I am guessing every little addition would eat away at the alternatives.
Moving from WordPress, the fact that you couldn't constantly tweak a thousand little things was extremely liberating. That's the zen-like simplicity of GitHub Pages that to me, makes it an attractive option over heavyweight alternatives. Just push and your site is live. Fewer things to break and fewer things to worry about means more time to focus on what matters: your content.
/article/foo => /pub/article/foo.html
Or something very similar... That, or a custom 404 map, to take old urls, and send them to new ones (in the case of a blog, for example)... that said, gh-pages works well, and it's pretty awesome that it's offered to so many floss projects, and in my mind reduces the chances of a custom domain/website going away because someone doesn't really support something they put out there 7 years ago anymore.