My least two favorite features:
1. Interactive notifications...well at least for mail. In iOS 7 I am able to be in a game, other app...whatever and get a mail notification. I would swipe down and read it, swipe it back up and go back to my game or whatever app I am in. In iOS 8, I can get that same notification and the only options I get are mark as read or delete. I can't read the notification without going into mail. What a waste.
2. The need to have message preview turned on to use quick reply. I keep my message preview disabled for a reason. Defeats the purpose of leaving the message preview disabled. Maybe if they would build an option to leave it disabled but when you use quick reply, then you can see it.