I'd like to filter so that I only see cards that are not labelled. I can see how to filter for any combination of labels except "only show me those that have no label". I think this would be very useful, but can't figure out how to do it.
You can use the Trello API not to filter but probably to solve your problem, if you have programming skills. It's not a complicated API to use. https://trello.com/docs/