I FIGURED ONE SOLUTION OUT! I have spent the better part of my Saturday trying to get my new iPhone 6 to sync with my gigantic iTunes library. It also got stuck in the process no matter what solutions from this forum I tried. Then I remembered an old work-around for missing album cover art and I tried it--voila! All my music loaded on the iPhone 6. Here's what you do:
1. remove ALL music from your new iPhone by unchecking "Sync Music" box.
2. Sync phone (it should remove all songs); even if it gets hung up on the last step, you should see all songs removed.
3. Restart iTunes and iPhone.
4. Select "Sync Music" box and songs/playlists etc you want.
5. Select "Sync."
This should do the trick. Mine is loading 8K songs right now after nearly twelve hours of trying to get it to work. Hope it works for you.