I think the recall system in libraries would be so much more effective if there was a way for you, the desperate researcher who needs a book like NOW, to directly contact the person who has the book out, the guy who may or may not be using that book and was just too lazy to return it. The library’s default message “you have 2 weeks to return your book” does not, I think, adequately convey the urgent nature of many of these requests. Also, you could work out better reward systems between yourselves (I’ll make you cookies!!) and, at the end of the day, even meet someone else who is into the same topics as you! It’s a win-win.
Of course, there would have to be measures taken for anonymity. Maybe a system like Craigslist currently has?