From [[:commons:VP]] (https://commons.wikimedia.org/wiki/Commons:Village_pump/Archive/2013/06#About_the_Upload_Wizard):
Most newbies also don't add any categories, because the upload wizard is not recommending it. If no categories is given, the upload wizard should be giving for example a warning in a red box or with red text that categories should be given.
We obviously can't make categories *required*, since they aren't required, but having better notification of the importance of the field would be good.
Proposed warning popup: