mrbenn is sort-of correct - In his scenario I do want to identify the other 499 tracks, but not because I want to create another playlist from them.
I too have in excess of 30,000 tracks and as the library grows, the problem increases. It's great to be able to create a smart list that might, for example, filter the library down to about 10,000 tracks which are then randomly played. This is great for digging deep into the library and plucking out random tracks that you wouldn't often think of playing when you create a manual playlist.
But when you put together a set of rules, the more creative you get with these rules, the harder it is to know what you might be eliminating. So from the 20,000 excluded tracks there might be whole groupings that have been excluded unintentionally - The point is, you'll never know what's been excluded because all you can see is a long list of the 10,000 'successful' tracks.
I know from experience that if I had the chance to flick through some of the excluded tracks I would very quickly find some that I would rather have kept in the playlist. Looking at any one of these tracks would make you think again about what rule had excluded it (and other tracks like it), and you would quickly realise how to tweak the rules to accommodate any changes.
You would then check the list of exclusions again...and after a couple more iterations of this process you should be able to get the rules pretty close to perfect.
The thing is, this would not be a difficult feature to add. The work has already been done, we have auto-playlists already. I just want to be able to flip backwards at the press of a button and see what I might be excluding.
Just to be clear, if all you want to do with your smart playlist is, say, "Find all tracks in the 'Blues' genre with rating higher than 3 stars" then this feature will never be needed. But some of my auto-playlists have more than a dozen rules and at that level it's really difficult to calculate in your head exactly what the effect will be on every individual one of those 30,000+ tracks.