AllMusic is amazing for listing all the possible moods, etc. for music and artists. It's so good that it seems like overkill in that if I start tagging my collection to that level, I'm going to be so micro-focussed that I'll never find anything.
For example, between playful and fun, I'm not sure I could identify the difference. I'm working on a list to tape up near my computer of a subset of those terms, but am curious to what detail others with reasonably large collections have employed keywords.