iTunes 11 brought with it a pretty hefty overhaul of the interface many of us have grown accustomed to. One of the more significant changes is how one searches an iTunes library. In the latest version, typing a search query reveals a truncated list of results below the search field from which to choose. However, many preferred the way it worked previously, filtering all items in the selected media category (e.g. music, movies, etc). Thankfully there's still a way to do this, and you can even retain the old way of doing it if you want.
iTunes' Current Search Interface
To temporarily search the old way, being by selecting the media category from which you'd like to search (such as Music). Then start typing your search query. To actually filter within the main window, press the return key (or enter on Windows). The search popover will disappear and the list of items in the main window will be filtered by your query. You can continue typing your query to further filter the results until you've found exactly what you're looking for.
"Search Entire Library" Unchecked
If you'd rather "permanently" switch back to the old way of doing searches, there's a way to do that as well. Click the magnifying glass in the search field and uncheck "Search Entire Library." Now whichever media section you're viewing will be filtered upon just as it was in previous versions. Enjoy!