Ability To Comment On A Block Of Code
Reviewboard (because I can't stop talking about it) allows you to do this. You click and drag on a number of lines and it'll allow you to comment on the entire block.
When the review is being read, the entire block is highlighted, looks nice, brings attention to the entire block instead of a single line.
Again one of those minor UI enhancements that may be on the drawing board but isn't here.