Primarily thinking about Play Count: it would be nice to be able to query or create a smart list using at the very least minimum and maximum values for the play count field.

Sums, averages, and other aggregates supported by the SQL backend may also be useful. It may be worth adding a common query syntax for handling arbitrary aggregates.

The primary use case would be Smart Playlist ideas such as (to replace/work along with the Unheard default smart playlist):

Least Heard: playcount=min(playcount)
Most Heard: playcount=max(playcount)

Easiest way to do this may simply be to use a couple of pseudo-fields such as minplaycount and maxplaycount, rather implementing a new query syntax.