Currently in MediaWiki, adding categories requires knowing obscure and esoteric code (as well as knowing which categories already exist on the wiki). This sucks. For geeks and other techies, it makes sense, but for regular people, they want to know why MediaWiki doesn't have a drop-down or suggestion system of some kind. And they want to know why they're required to manually type out the code and full category name, when plenty of other sites have features such as textarea tab-completion or auto-suggest or whatever else. The regular folks are right: a proper interface for adding categories is missing currently.
One option to resolve this bug might be to examine integrating something similar to HotCat (http://en.wikipedia.org/wiki/Wikipedia:HotCat) into MediaWiki core. Other options will require more thought and development time. This is a serious interface issue, however, and I can't find any duplicates after a quick search of the database, so I'm logging this for now.