The search form requires you to input a keyword. The problem with this what if I want to search for, say, all posts by user X in forum Y. I can't seem to do that. I was going to post this as a bug, but was a little undecided, so here it is as a suggestion.
Note that this refers to the Advanced Search form. The navbar search form allows you to search for all posts by one user, but you can't specify a forum.
It is actually intentional. It can be hard to come up with a reasonable criteria for what's limiting enough, so I figured the query would be important. There's a special case for "find anything by this person" and it just redirects to that. It's probably reasonable to include the forum limit there as well.