* Push conflicted branches so developers in a distributed team can work together to resolve conflicts.
* A git-powered version of Rake, which runs only those tests that need to be run based on the git history since the last run.
* A tool to identify what code changes caused a particular test to fail, based on the above.
* Language syntax detection for smarter diffs, improving the display when blocks of code are moved around or indented/outdented.
* Language linters built in, to detect when a change is introducing a syntax error. Same for coding style.
I'd also move the "Close Pull Request" button a little farther away from the "Comment" button, and make it possible to add comments to a diff when you've hidden whitespace differences (with `&w=1`... I'd also make it a bit more obvious that you could do that).