In the category search form you can go to each category and click to include it or exclude it (advanced option). The form also has the categories set into reasonably defined groups that make a lot of sense, in most cases.
What I'd like to see is an addition on the Advanced form to include the + / - tick boxes in the group title. This would allow a person to click on that box and it will then fill in the boxes for everything within that category with the same tick. So instead of having to go in and tick every damn category for that group I just do the one tick and it's done. I'm sure this will make it easier to set out a search, especially the exclusion searches.