It would be nice to know how long the user spends between receiving their search result and clicking through to a result. It might give some indication to the quality of the results, users clicking through faster saw what they were looking for immediately, while users taking longer might indicate they scroll through the results, consider a few options, then click one that is maybe kinda/sorta the right answer (or not).
Collecting these in javascript might get harder, it would be nice if we had a way to collect this information from the webrequest logs directly. This might tie into the previous idea of giving each search a unique hash, and having click through's from search hit a redirect bounce that includes tracking information and then redirects them on to their final result. We can't necesarrily include these extra tracking parameters in the link to the article itself because that would hurt article cache hit rates (unless we stuff all the data into wprov somehow)