This is something that would add to the significance of the "Done" button in that users could specify whether there comment is stating a big problem (unbreak now/Fatal), will cause a race condition(Major), a typo or syntactical issue(Minor) , etc.
Basically this would allow for reviewers to "submit issues" to the author.

Technically the IMPORTANT, WARNING, NOTE syntax can be used now simply for presentation purposes but it doesn't aid itself well to any forms of statistical representation.

This method could use a drop down that would default to "comment/note" and the main significance this would have over the above mentioned syntax is that you could have a field/table that would state how many Fatal, Major, and Minor issues have been reported against this review and even using the "Done" feature you could have it show either how many issues have been marked "Done" or how many of the reported issues are left.

This type of statistic is often used in the managerial sense to gauge how long a piece of code is going to take to make it in based on the amount of issues against it. This is also helpful to the author at quick glance to see how many comments of concern were raised without having to look through the entire page.