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

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...