As usual I warmly welcome any questions/criticism anyone has. Part of the reason I love HN is because the feedback you get on ideas is instantaneous and incredibly useful!
You made the visual counterpart of the search engine NOT operator.
Google:
"blue suit AND women -men"
Yes, exactly. The problem with the standard NOT operator ('-') is that it's really hard for a user to discover it by experimentation, therefore most users never find it.
By exposing the same functionality in a visual manner it makes it much easier for the average user to play with...
One additional point to note is that if you show the user which keywords/tags are associated with the entire result set they are much more likely to be able to intelligently, and quickly, weed out results they don't want. As opposed to having to scan through the result, find the string they don't want, then type it into the search bar preceded by the negation operator...