“Control, exploration, flexibility, tunability,” were the answers expounded by representatives of Microsoft, Endeca, and Vivisimo. Relevance is in the eye of the beholder, but relevance ranking is driven by the search engine. Know what criteria are driving the ranking of the results you’re looking at, or at least, be skeptical of them.
I couldn’t agree more with Theresa Regli’s excellent discussion of relevance, especially the point to be “skeptical” of why results are the way they are. This is definitely true for search application developers too. The problem is, if you’re using a proprietary vendor, the only thing you can do about such skepticism is bang your head against the wall. For Apache Lucene and Solr, on the other hand, understanding why something scored the way it did is as simple as making an API call (Lucene) or adding &debugQuery (Solr) to your input and you get, in full unadultered glory every last detail about why a particular document scored a particular way for a given query. Furthermore, if that doesn’t satisfy you, just pop open the source code or ask us or ask on the mailing list!
At the end of the day, it’s your data and your users, why shouldn’t you know all there is to know about your results?