One thing I just noticed that GitHub added at some point is the ability to change which branch a pull request is targeting. In the past, if someone contributes a pull request but we prefer to target a different branch, it was a pain to adjust it. Now, it's a simple matter of using the Edit button at the top (as if one were about to edit the title). I presume that GitHub project administrators and the submitter can both easily do this.
It's a small thing that's pretty handy.