Ok, I guess when the ID3v2 tag support gets put in, it should be able to build playlists by whatever criteria you want. My previous example was not too good. How about if I want to play fast rock, or fast techno, or slow country (ya, right), but whatever, or maybe fast rock by The Tragically Hip, or slow rock song by females. This is what I am getting at. I hope that in the future this will be built in, as it seems like a pretty simple way to do things. This way, as long as all your tags are filled out, you might not need any pre-made playlists at all.
You just select New Play list, pick your first criteria, be it genre, tempo, year, artist, album, whatever, then you pick the 2nd criteria.
This may sound a little confusing, but if you see it in actions (like in Music Match Jukebox) it is really easy, simple, fast, and extrememly useful.