At the moment, you can set up your Smart Playlist rules and choose whether media files match all the rules or match any of the rules. My most common use for Smart Playlists is to capture new episodes of series I am watching. So my rule set for a playlist might be:

Play Count is Zero
AND
File Name contains Silent Witness

But the fact that you can only choose to match all rules or any rules is limiting. What I'd actually like to do is choose different AND/OR combinations within a rule set (I suppose you could call it full boolean search). An example is the easiest way to explain. For example, I want a single smart playlist that allows me to capture unwatched episodes of different series, e.g.:

Play Count is Zero
AND
File Name contains Silent Witness OR File Name contains Sherlock

I hope that makes sense.