According to studies, the effect of code peer review is a factor of the time spent reading the code, the number of lines in the review. For example, some studies say reviews should not comprise more that 200 lines of code.
If the results of these studies are consistent, it would be nice if Crucible took those (configurable) criteria and displayed things like "your review contains more than x lines of code which may reduce its effectiveness" or "Crucible recommends you spend x minutes on this review".