An effective critique is one that will show both the student and professor the extent of what was learned by the student who did the work. It is assignment-specific, focuses on the criteria for excellence outlined for the project, and contains justifications for opinions that are put forth.
Crits themselves are forums for learning new things, and an effective one will leave all concerned feeling like they gained new insight into the creative process. Additionally, a useful critique will point the student in new directions, suggest new paths for exploration, and give the teacher important feedback about the relative success or failure of the assignment itself.
Finally, an effective critique is one in which there is clear respect shown by all participants. Students and faculty will leave such a review feeling energized, focused, and excited, instead of exhausted, confused, and depressed.