It would be much better if the filtering system didn't 'hide' the tags that were outwith the current selection (So you could tic on "trees" and "people" at the same time.) - perhaps just change the colour of the non-relevant tags?
It would also be nice if the tags had three states: (none/ignore), Include, Exclude
And if there was a way to remember (load/save?) a set of tag filters.