Thanks for all the input, guys. I guess the features I care most about are:
1. Super-tagging (I rate songs and give them weird genres, often I even store lyrics in the file)
2. Auto-DJ based on info in the tags
3. Convenient/Easy GUI for displaying tag information
I also like having album cover art, CDDB support, and so on, but they're less essential. I haven't found any other programs that support these capabilities. If I'm just missing out (I didn't thoroughly go through the documentation for Foobar or Quinntessential), let me know. Otherwise, I'll report back about anything else I find out.