Tagging is the first step, but how do you know if you don't have overlapping or duplicate tags, say country and folk music? If you need something that fits both categories, you eventually start designing taxonomies and eventually ontologies, there's just no end to it. I think tagging is a sensible, lightweight approach, but it has limitations...
Folk music has nothing to do with country.
Country was invented in the 1800s by a businessman who aimed it at primarily white Americans. Folk music has a rich history dating before the 1200s and before much of written language. Many songs of Irish and Welsh descent date before record, still being played today.
Nevertheless, to answer your question:
1) metatags, parent tags, and the like provide ways to structure tag relationships to encompass and describe what you speak of.
2) There are many networks out there that share user's tags based on file hashes. A system could scrape these networks for existing tags and autotag many items without the user's interaction (beyond initiating the autotagger). Users could also get their hands dirty and ask for only tags relating to parent categories, or something like that.