It would be nice if we could click on a user to list all their posts then be able to use the search engine to further narrow down the results. An HP forum users was looking for someone's post on the essay detailing the parallels between Hitler and Bush and all they had to go on was that a certain user posted it, and made this suggestion.
Watch for the 6.4 changelog coming out pretty soon. The search engine has been rewritten with some new options including the ability to search for certain keywords in posts by a certain user.
While that's great and all (and it is!), anyone think this sort of functionality would be worthwhile to have while we're waiting for 6.4?
There are hacks to the search page for search by username as well. Look for SearchPage Upgrade Hack 3.0 or something. There are several different search page hack, so make sure it has the functionality you want.