After doing a quick and dirty flash over the top of the first preview (so I didn't lose my user data), I'm surprised by how few changes have occurred. The app drawer is nicer.
That's about it. I'm not 100% sure as I was only in a 4G area briefly today, but it looks like it's showing an LTE icon instead of 4G, which is a bit odd. Also, Bluetooth is somehow even worse. It's almost like no one bothered testing it, as it constantly crashes. I could put up with the odd glitch when streaming audio to a speaker, but now it crashes almost immediately
. I've had to resort to using my Nexus 10 in order to stream music from Play Music to my Bluetooth speaker. Thankfully I use an auxiliary cable in the car!
Which is a good thing, as I tried listening to BBC 6 Music for a couple of days on the way to and from work. It's pretty awful. The hosts were terrible and I could regularly hear compression artifacts. Apparently 6 Music uses 128kbps MP2. Yes, MP2, not MP3. Not that I'd be happy with that bitrate for MP3. I'll stick to playing songs off my phone, falling back to streaming other songs using my phone's data connection (as I get unlimited bandwidth).
It seems that FM and an aux cable is better than DAB and Bluetooth, which is quite depressing given it's 2015.