"Diffing the commits" isn't really available in in a GitHub-style Pull Request web UI, which is where 99% of our code review is happening. I'm definitely optimizing for that view of the merge over everything else.
I love how github fosters discovery and remote collaboration, though one of its liabilities is when great git command-line features are effectively lost unless github re-implements or exposes them, because some conventions incentivize only doing what github itself can do.