My itunes wouldn't sync to iphone, either. I couldn't get my playlists to copy from my macbook itunes to my iphone 4s. I always "sync selected playlists". This time they showed up on my macbook screen listed as if they had copied to the iphone, but the tunes were greyed out on the mac and the iphone didn't show any of the changes. Didn't matter if I checkboxed-and-sync'd them or direct-dragged them to copy them: nothing budged.
Johnf70's advice solved it for me, too:
"Try this: Plug in your phone and open iTunes. On the sidebar click "Your" iPhone > Summary... go down to Options and uncheck everything. Then sync. By doing that all of my music was restored to my phone. Never had issue with my apps so that was fine. Afterwards I checked "Sync only checked songs and videos", resynced and everything was back to normal... for now."
One change: I actually left "convert higher bit rate songs..." checked, just to see if I could stay with that option (more tunes, less space) and that worked fine. All my chosen playlist songs -- the ones I had already checked as "sync selected playlists" under the Music tab -- began copying over when I hit "Apply settings/sync." (I'd already backed everything else up, because I'd been trying to sync for an hour already, so it went pretty quickly.)
I looked at lots of more complicated solutions in other threads involving icloud, match, double rebooting, and whatnot. I'm really glad this straightforward one worked, even though I don't know what the problem is (darn you, apple!) or why this solution works. Thanks, Johnf70!