This might be a known thing, and I didn't know it because I wasn't paying good attention, but
I finally upgraded to MC18 last night. Overall it went okay, though I did have to fight with my GMLs a bit to get the remote controls working right (as expected).
However, one thing kinda stunk...
A few months ago, I set up a very clever system using a bunch of Play Doctor "lists" to populate some views and other smartlists, which I then sync over to my phone and serve to JRemote/Theater View via View Schemes.
I keep these in my Playlists folder in two different places:
iTunes/iPhone Specific Lists: Playlists\iTunes Lists\Play Doctor\
Full Library Lists: Playlists\Play Doctor\
I have almost-duplicates of all of them, because the ones for iTunes are limited to the set of files that would already be likely to be synced to my phone (it limits Play Doctor to using only the contents of the lists that are synced). That way, I can add these "shuffles" to my phone, and they don't eat much capacity on my mobile device. It works great, and did after the MC18 upgrade.
However, ALL of my custom lists in Playlists\Play Doctor\ got blown away/erased/deleted (or maybe moved somewhere I couldn't find them). At first, I thought I was crazy, but I loaded up my copy of MC17 (which was using a backup library cloned just before I did the upgrade), and it was fine. Only the lists in that default Play Doctor folder got nuked. Any I'd moved elsewhere escaped unscathed.
Fairly annoying. It took me a couple hours to fix it. Recreating the Play Doctor lists was easy, as I just had to copy/paste the ones from the iTunes Lists folder and tweak each of them (they were identical other than that extra limit). But I had a bunch of Standard, Theater, and Media Network views that were "linked" to these Play Doctor lists, and I had to go through each one and fix them to point back to the proper lists by hand (the Playlist is X rules all got blanked out). I have 12 of them right now, but I'd planned to do 10-15 more. Good thing I'd been lazy. If I'd had 20-30 lists to fix in three places this post would have had a different tone.
Nuking playlists or parts of the Library without warning isn't cool.