I think the default is if you enter one word, it automatically searches as a keyword. If you enter several words, it searches as a title. Or if you enter “by:” it searches by poster. You can also enter “keyword:” to search by keyword or “title:” to search by title. I also noticed that if you leave it blank, it brings up a page with the regular search by title box.
You're right... there's the old search box with the drop downs that we were used to!