I agree about anonymity. I'm a high school teacher and there are a bunch of smartphone apps that let students send in answers to multiple choice questions that the teacher puts on the board (I think it's called SmartResponse). The problem is, all responses have the students' phone numbers attached. So privacy concerns make it so that an otherwise useful product is basically unusable. Obviously the level of concern would be different in College/University, but I think students might be less inclined to use the app if it is not anonymous.

That said, this is a great idea. This is the kind of real-time feedback that I would love to have access to. Many of my students would rather fail than put their hand up and ask for help. Something like this would really help them out.