Let's posit that there are two ways of information search/seeking: the directed searcher, and the explorer. The direct searcher knows what he/she wants, either a narrow search or in broad swatches. Then there is another behaviour which is the explorer. When users are in this mode, their minds explore what is possible and their eyes roam to what attracts them.
Understanding these two behaviours, interaction design becomes quite a simple problem to solve. In the former, the interaction design is to get the user/reader to the target object as fast as possible. In the latter behaviour, the interaction design is to get users to stick around your system for as long as possible.
The best interaction design I have seen and experienced for the former behaviour is Google. The whole design of Google is to get people off their page as soon as possible.
Ironically, the best interaction design I have seen for the latter behaviour has the most minimal of UI design. I'm talking of course, about StumbleUpon.
Unfortunately it is very difficult to monetize SU's interaction design (SU ultimately found a method to monetize its assets which I found rather interesting). The Pinterest UI is merely a logical conclusion of designing interaction for lingering souls and wandering minds.
Imagine how easy it would be to have a 300x250 image ad in one of the boxes. Of course, the surprising thing is that Pinterest does not actively use display advertising to monetize.
[0] http://theforkingchef.com/post/41947940039/search-behaviour-...
[1] http://theforkingchef.com/post/42355830855/the-subscriber-an...