Let's see:
- random photos missing from my photostream on other devices
- photos that reappear in photostreams after you delete them
- files in iCloud enabled apps that refuse to sync (stuck on Waiting forever)
- calendar invites that keep coming back after accepting or ignoring them
- iTunes match unreliable and slow (seems to be better with iTunes 12)
- unable to easily backup iCloud files
- reminders, contacts, calendar out of sync
- iMessages reported as Delivered while they are not
- FaceTime devices not ringing when you call them
- apps that rely on iCloud sync hang in ios 8
- slooowwwwwwww (yes, still as slow as iDisk or MobileMe)
not a bug but horrible design:
- the need to delete photos both in the camera roll and photo stream
- the iCloud website looking like a fisher price toy
- the app specific folders in iCloud drive
- the inability to share iWork documents in the two months leading up to the release of iOS 8. Because they were working on a new version. Really?? It didn't get a lot of attention, because, let's face it, nobody uses that feature.
- etc