Another vote for some sort of rating system.
I'm another media monkey user who'd like to get away from that and use XMPlay more. I use XMPlay at home to **listen** to tunes and media monkey to **manage** tunes. In other words, I create playlists, sync my iRiver player, fix tags, etc., on MM. But MM is way too clunky to use as an everyday player, and it's not as customizable, so I happily play my music with XMPlay. I don't want XMPlay to get too bloated, so I'm willing to live with that system.
So to my mind, any functions that enhance my ability to listen to music--like quickly finding and playing a song, quickly making a random playlist, rating a song as I'm listening to it--is worth adding to XMPlay. Anything that gets into the territory of music management--like having a massive customizable database, converting tracks from one format to another, syncing with a player, etc.-- should be left to a music management program.
The tough part, of course, is figuring out what belongs in a player and what belongs in a management program.