So, as you might have noticed, I finally upgraded to a new iMac 27-inch with 5K Retina display recently.
While cleaning up my data for the move from my 2009 17-inch MacBook Pro, I decided to switch the email assigned to my iCloud account. No big, right? Well, I haven't had any of my iTunes playlists on my iPhone since I logged out of iTunes and back in. Try as I might to figure out why my playlists wouldn't sync, I was getting stuck.
Until I decided to search the Apple support knowledge base for an answer. (Article HT204406 fixed my problem.) Turns out that the problem had nothing to do with iCloud, wonky Wi-Fi connections or an alternate iCloud email address.
No, the answer was 100% user error. The fix, a simple one, was to just turnoff the "iCloud Music Library" sync option. D'uh!