The ability to only search the first post in a thread rather than entire threads would be handy.
*Makes it easier to find a specific topic. For example, the word "space travel" might come up at some point in every thread, but limiting it to the OP keeps it down to threads specifically about space travel.
* Less load on the server. Searching ~50k posts to find what you're looking for instead of ~5 million would lower load, and make it feasible to lower the search delay for that type of search (for refining search terms).