Being able to for example, send a picture of a book knowing I'd get one back would be pretty nifty. Food would be a popular topic for swapping pics judging by the main page.

Well, unless you want to use geolocation and match them up by location. Think of all the romance you could spark of you knew what sort of genitalia people sending were hoping of receiving.

joking aside, while i agree with your model, i'd be curious to see cases where users get trapped in these auto-identified sub-worlds of obscenity. I imagine as we get better at content recognition this model will become more prevalent, and thus innocent crossfire will be more commonplace (even if to the tiniest degree).

Interesting times we're heading into