I think all search results would be improved by the ability to exclude departments. Instead of forcing me to guess whether an item has been filed under “Home & Garden” or “Grocery”, let me instead exclude “Baby”, “VHS”, “MP3 Downloads”, and “Clothing & Accessories”. You already allow negative keywords in the search field, so this would be a natural extension.
This would be particularly useful for your “recommendations for you” list, which in my case is currently dominated by cookbooks, salami, and SF novels. If you want to sell me anything else, you have to give me a way to sort it to the top, and the current positive filtering system is trial-and-error, since most of the listed categories don’t actually have anything in them to recommend.