How about a whole new level of complexity?
There are currently several hierarchies - genre, style, artist, album, track...
It would be nice to have a way to rate or assign a weight to every level. For example, you could have a rating for your genres:
rock = 5
jazz = 4.5
electronica = 4.5
new age = 3
classical = 3.5
country = 2
etc.
(that's just an example, not necessary my weights)
Then you could also assign a rating to every other level like style, artist, and album (we already have ratings for tracks).
The other feature that I think would be cool, would be "listeners". When you are running MC and listening to music, you sign-in as a listener. And, you can have more than one listener at a time. When playing music, all of the play history will be associated for each listener. And all of the listeners will have their own ratings (at each level).
Then you could have a smartlist like:
Listener1 genre-rating > 3 AND style-rating > 3 AND artist-rating > 4
Listener2 genre-rating > 3.5 AND ...
Listener3 genre-rating > 4 ...
Each listener could be named, so in my case User1 would Gregg. When a listener signs in, then the smartlists will only use their settings, but if other listeners sign in, the smartlist will do a boolean AND to include their preferences.
With this kind of feature, you could have an extremely powerful "family" or multilistener system.