I regard categories as far inferior to topics lists. To repeat for the zillionth time (OK, maybe the tenth or fifteenth in almost five years I've been a registered user here):
- Lists can be organized; categories cannot. See, for example, the list of geometry topics, the list of probability topics, the list of abstract algebra topics, the list of complex analysis topics, the list of real analysis topics, the list of algebraic geometry topics, list of computability and complexity topics, the list of number theory topics, list of numerical analysis topics, list of probability distributions, and many of the other lists listed at lists of mathematics topics.
- And see lists of mathematics topics, a featured list, chosen as an example of Wikipedia's best work.
- Lists can be annotated; categories cannot (see e.g. list of rivers by length).
- Lists can contain invisible links to talk pages, which cause edits to those pages to appear when the user clicks on "related changes" (thus: [[Talk:ABCology | ]]); categories cannot.
- Red links to not-yet-existing articles can be added to lists but not to categories.
- Alternative names for the same item can be added to lists, but can be included only by including redirects in the category.
- Lists can give context or commentary for any specific entry; whereas in categories only the name of the article is given.
- The history of the membership of an item in a category cannot be determined; with a topics list it can.
- Categories can only use alphabetical order; they cannot use historical chronological order, or order from biggest mountain to smallest, or ordered in some way that is natural for the subject matter (have you ever seen the months of the year in alphabetical order? April August December February January July June March May November October September).
- Lists can link to specific sections inside other articles.
- Lists can be moved. You can't move a category containing 400 items except by editing all 400 articles separately. Moves have happened with lists of mathematics topics and list of mathematics articles, and the reasons for those moves had to do with the evolution of Wikipedia over time, in ways that could not have been foreseen when the lists were created. And the number of items in the list of mathematics articles is not just 400; it's many thousands.
(I'm probably missing some items in this list. See Wikipedia:Categories, lists, and series boxes.)