Faceted Search and Query Reformulation
One interesting study Mark Baker pointed me to is Incompetent Research Skills Curb Users' Problem Solving on the Nielsen Normal site. Nielsen found that searchers are becoming so trusting with search results that they assume if they don't find the answer immediately with the same type of search query, the answer isn't available. They don't try different search strategies to try to surface different results. Nielsen writes,
Still, the rough estimate from our available data is obvious: users change search strategy only 1% of the time; 99% of the time they plod along a single unwavering path. Whether the true number is 2% or 0.5%, the big-picture conclusion is the same: users have extraordinarily inadequate research skills when it comes to solving problems on the Web.
In other words, users are poor searchers. That's because all most sites provide is a little white box. I assume users try different combinations of the same general keywords. But faceted search provides a way to intelligently filter the results, which indirectly creates a more advanced (or different) search query. When you search and see all these filters on the side, you can perhaps see the correct vocabulary to use. If you get too many results, you keep selecting filters until you get a manageable set of articles.
For example, in the gamification context, a search for "progressive levels" might not contain the right answer, but one facet that will appear is "mission." A mission is a series of tracks, which are collections of achievements. Users probably don't know all the "right" terms with their searches. Instead of typing every synonym they can come up with for "progressive levels," they can instead look at the list of facets and more quickly carve the results down to the information they want.
Neilsen doesn't say whether the searchers had facets at their availability. Most likely they used Google, which does have some facets (web, images, maps, shopping, videos, news, books, blogs, flights, discussions, recipes, applications, patents), but these facets aren't focused on content and meaning. These facets are useful in a general way but not for the situation I described. They don't narrow down to "mission."
In this way, facets that focus on content and meaning might be more powerful than other types of facets.