I liken this problem to browsing a library or bookstore vs searching its card catalog or asking for specific information. If I have a problem, I know what I want and it may not be something I've wanted before. If I want to relax and browse, I often want something different than I've wanted before.

So I think it is a hard problem, as said here, serendipity is hard to program. However, I am absolutely sure we have not yet figured out the best way to browse. Search seems pretty good, but internet browsing can easily be too time consuming and aimless.