Have read this entire thread after my smart playlists stopped syncing properly, suspiciously immediately after the latest iTunes update. Tried everything in this post from restore of iPhone to all the tips and tricks about the smart playlists. Here's what eventually worked for me:
I rolled my computer back to my Acronis weekly backup from last weekend, only a week ago and the computer is really a HTPC so didn't lose anything really. My main aim in doing this was to get the older version of iTunes back as I was convinced that's where the problem was.
After getting that all done I got my playlist I sync no problem and thought "problem solved!" That was until I edited one if the smart playlists to exclude one star songs from the list and wouldn't you know it, blank playlist on the iPhone again! Removed that condition from the smart playlist and voila! All working normally again.
I trawled this thread again and found what fixed it for me was what someone else had posted (apologies I can't remember who and can't be bothered to trawl again to link to the person but kudos to them).
All I did was add the condition to the smart playlist to select from the playlist Music' and everything synced over again. This would really make it seem like iOS7 has an issue with some smart playlist parameters as reading the thread many people seem to not have had this issue on iOS6. Regardless, it's bloody annoying considering the whole reason I got an Apple device was I wanted a system for getting music on my phone that would just work, I mean that's what Apple cut it's teeth on in the portable device arena.
Anyway, it all seems to be working again now but I'm not going to upgrade iTunes until I've got another backup tonight and even then I'm leery of doing it. What a palava!
All this on iPhone 5 and iOS 7.03
