We've got a great user video section. It's devoted to helping people improve their videos, and many users register for this help alone. The best way of doing this is to provide a critical assessment of the video, but I think there's a bit of a confusion over whether a critique is "constructive" or helpful. In my opinion, any critique is useful, as long as it's reasoned. For example, "I stopped watching 10 seconds in as I was offended by the language".
What do you take constructive criticism to mean?