XnView Software

I know that you can filter pictures by category by selecting the ones you want in the Category Filter; however, this is sometimes buggy and doesn't allow for much customization. What I'm proposing is a full fledged, text-based category filter, where you can type the name of the categories you want to display, using logical operators (AND, OR, NOT) to combine them in multiple ways. In my opinion, this would allow for a much more complete browsing experience, and I would love to see it implemented.

You're not alone... see this and following posts. Hopefully, Pierre will rework this part for one of the next major releases, together with the XMP/IPTC handling.

The GUI paradigm used by most other programs are is column with tri-state checkboxes (gray = doesn't matter, checked = must be, unchecked = must NOT be) controlled by a global drop-down menu (which already exists) with „Match Any/All”.

See a proposal for XnView MP here:
http://www.xnview.com/mantisbt/view.php?id=389

(this will make the program simpler because it will be not needed anymore to have two trees to manage).

In the Search dialog which is more advanced, we have already a tri-state check-box so you can search there for NOT IN. However, indeed, there perhaps there one would like to have some AND and OR operators, but I really don't know how „user-friendly” there operators are for the Average Joe User.