I would like to see an auto-complete feature in the search bar, similar to AnswerHub, uservoice, and other services with similar UI's. I don't think this feature is available today.
If I start typing a quesiton, I would like to see the results (preferably categorized), before I press enter. For example, if I have a category called "Products", a tag called "programming", and a question/answer with the word "Prolog" in it, I would like all three to appear in an autocomplete list below the searchbar, when I type "pro", each under the appropriate heading ("Categories", "Tags", and "Questions", respectively), along with any other matching results.
This sort of responsive UI has become table stakes with modern UI design principles, and not having it is noticeable.