One of the big problems for a speaker is getting good, usable feedback for your presentation. You won't get it directly from your audience after your talk - people are too polite. Would you go up to a speaker and tell him or her that the presentation sucked and that you want your hour back? No, you wouldn't. If it was that bad, you would just leave the room and forget about it.
People are willing to argue with the speaker about certain aspects of the content but they don't usually comment on the form or the overall effectiveness of the presentation (e.g. whether or not it met their expectations). The most reliable feedback, in my experience, ends up in the feedback form (electronically or on paper) that the event organisers prepared. And even then you need to motivate the audience to fill those out.
So it was interesting to see a new (to me) approach being tried out at a conference that I attended recently: Feedback by coaster.