The tag system is fine, and the names/explications of the tags clear enough. But why is there no way to add facultatively primary (and secondary?) AMS classification numbers, like in many journals?
I don't mean replacing the tags, but as the order in which the tags appear for a given question is rather random, the primary AMS classification might especially be useful for sorting or searching purposes. The majority of tags in the thousands of already existing questions could be automatically converted into corresponding AMS numbers, though that would not give a hint about which one should be considered primary for a given question.