Quite often when reviewing code I will make a few comments on the diff. Some of these comments are important and block me from clicking 'Ship It'. Some of the comments are recommendations or suggestions. At the moment I have to follow up these diff comments with a review comment, pointing out which issues I expect to be fixed and which I don't. It would be nice when adding diff comments if the author could set this as a 'blocking' comment and when someone is reviewing the comments the ones set as 'blocker' are highlighted to indicate their importance (surround the comment with a red box for example). Obviously I could add an additional line to the diff comment saying I expect this to be fixed, but an extra highlight would make this more obvious to the review creator.