I often want to type a code example in response to a diff (e.g. "consider doing it this way!"), but <textarea> is a pretty awful interface for writing out code. It'd be awesome to have an editor which could at least keep an indent from one line to the next.
Possibly worth a separate issue, but I've always wanted a "Suggest Change" feature where I could just edit the reviewed code directly and the author would have a button (or arc command) to accept the suggestion and patch it into their local copy.