(Specifically <Keywords>, but others too if relatively easy to implement)
Click on a word within a tag to filter all files containing that word within the tag. Multi-select capable would be a nice plus, too.
For example, in a semi-colon separated list of <Keywords>...
female; jazz bassist; bassist; singer; songwriter; composer; Boston; Massachusetts; United States; Caucasian; African; multi-racial; American
This could work by linking to the "search" bar, perhaps? Using the above example, if I click on a word contained within the <Keyword> tag, it would be equivalent to Search Specific Fields... -> Set Specific Fields... <Keyword>, entering "female" and "composer" into the search bar and then executing the search.
A clicked word would be highlighted when activated. Clicking a highlighted word would remove it from the filtering process. CTRL + Clicking a highlighted word and/or a hot key would clear all filtered terms.