I like the backend changes the most.
Apps can now read from notification center
Much improved background task processing
Much better BT host app background task handling
This means realtime pushing of notifications to my Pebble and car status logging through OBDII port using Automatic. I never having to touch the host apps that feed or consume the data from these attached devices.
I'm know that I am in the minority, but I hate that they buckled and added Control Center with toggles (admittedly the other tools in CC are handy).
I find it lazy that the developer of an OS is passing off the responsibility of radio management to the end user instead of addressing of why users frequently need this access in the first place. I find it crazy that this is the type of "advancement" that people are looking for in an OS.