I have been struggling with this all weekend (first world problem, I know). My first attempt to fix this resulted in approximately 25GB of music being reported as "Other" in iTunes, which was over the phone's capacity, and in order to get back to a phone that would actually sync I needed to restore iPhone. Prior to this, I tried restarting iPhone, my Mac, turning off music sync, and a voodoo chant. Nothing worked.
After restoring, the "Other" was gone after the initial sync, but the phone still believed that it had transferred Music to itself. Very few of the 25GB of songs were actually transferred.
The suggestion about enabling Sharing on iPhone didn't work for me -- selecting my Mac's iTunes library on the phone just allowed me to play that library through the phone; it didn't actually transfer it to the phone.
What finally worked for me:
1. In iTunes, deselect any playlists that were syncing with the phone, and then resync.
2. Go to Settings>Music on iPhone and deselect "Show All Music". This sets iPhone to only display music that's actually on the phone -- not stuff that you may have purchased but not downloaded or stuff that's sitting in the cloud.
3. I opened the Music app on iPhone and pressed play. The phone started to play through the library, immediately skipping any songs that were not actually on the phone. Out of approximately 2000 songs, fewer than 20 were actually on the phone, so I just sat there while it blasted through the library in five minutes or so. When that process was complete, the Music app reported an accurate count of the number of songs actually on iPhone. Sync in iTunes and note that the bar in iTunes that shows what's on iPhone will change to reflect how much music is actually on iPhone. For me, it went from 25GB to less than 1GB. (Note that when this step starts, iPhone still thinks all the music that was on the phone is still there, despite being told to not sync it, from step 1.)
4. Re-select playlists in iTunes, then re-sync. At this point, iTunes and iPhone seemed to make up and the music actually transferred. I tested this with a couple small playlists by syncing and then stepping through the playlist on iPhone to verify the music was actually there. (Full disclosure: I did not fully step through the 2000+ songs on my phone, but I think they're all there.) Music app reports song numbers that are consistent with the playlists in iTunes.
Your mileage (or kilometer-age, depending upon your geographic location) may vary, but it worked for me.
Would be absolutely lovely if it worked the way it is supposed to out of the box. Not sure if this is an iPhone 6 problem, an iOS 8 problem, or some combination of both.
For what it's worth, if Apple is listening, here's what I'm running:
- iPhone: 6, 64GB, 8.0.2
- Mac: iMac, early 2010, 10.9.5
- iTunes: 11.4 (18)