Special:Categories hides existing categories currently having 0 members.
This has has several drawbacks:
1. When Special:Categories is used to find an appropriate category for a new article, and a category page has already been set up explicitly for this article type, it is not found. → Authors 'invent' their own categories. → Goodbye orderly development of a wiki!
When Special:Categories is used to find an appropriate category for a new article, and a category page has already been set up explicitly for this article type, it is not found. → Authors 'invent' their own categories. → Goodbye orderly development of a wiki!
Categories meant for QA purposes tend to be temporarily empty, when all pending work is done. While it may be nice to see to-do-lists shrink, it is counterproductive, when you do not see where to place a new QA job.
3. In wikies using multiple languages, authors very often insert category tags in the language they actually use, resulting in various duplications of categories. (Same applies to misspelled, and variant-spelled category names) During cleanup of such situations, it is vital that one gets a complete and comprehensive list of all there is, most specifically already created category pages.
(Currently, you need to have several windows of Special:Categories open (in chunks of 5000s sorted top down) and even more Special:Allpages (in chunks of 200, sorted 3 per row left to right) → finding related entries, or dupliates, is a real pain in the ass. Excuse me. I want to say, it's frustrating, and consuming much too much time)
4. There are likely more problems, which I just did not enocounter up to now.
Suggestion: include all existing category pages in Special:Categories, even if there are 0 members to the category.
Enhancement suggestion: If there is a justification for the current behaviour of Special:Categories, make it selectable by user input. In this case, I'd suggest to add a "show only categories with more than [ ... ] members" selection, which likely comes handy to avoid the disturbing 5000 limit sometimes, too. If you want to make it more comfortable, add another selection and make it: "show only categories with more than/less than [ N ] members"
Hint: If the 0 members problem, at least, is avoidable with the upcoming report extension, and we will be able to use the report extension in the Wikipedia installations, I for one would not mind waiting for it, and not pursue this bug any further.