I have two feature requests.
1) Please add the possibility to assign exactly one category to a set of images, i.e. remove them from all categories and then add to a single category in one go.
2) Please make the category filter sticky (possibly optionally with a checkbox).
Rationale: these two features would streamline a very common workflow when organizing a large and uncategorized collection. One could start by adding all media to Uncategorized, then set filter to Uncategorized and start selecting sets of images and "sending them away" to categories. Feature 1 would prevent the images from disappearing from view after the now-necessary intermediate "Category remove" step, while feature 2 would effect their disappearance from the filtered view once they have been categorized.