(1) Wi-Fi drops out frequently. (2) Battery life not as good. (3) Keyboard takes up half the screen.
That's all I've noticed, so far!
Odd because for me Wi-Fi has been rock solid, of course it was on KitKat as well. Battery life is slightly improved and my keyboard is currently taking one third of the screen in portrait mode.
Which keyboard are you using? I have Swiftkey and the Google keyboard and they are exactly the same size. In landscape mode the keyboard has always been huge, nothing new with Lollipop.
By the way SwiftKey allows you to change the keyboard size. I like it slightly larger for my meathook hands.
Performance is greatly improved which was expected with the move to ART. I wonder if some people's problems are due to apps that are not entirely ready for the new runtime.
My only quibble is the new Smart Lock Trusted Locations feature often doesn't recognize my home location so I have to enter my pass phrase most of the time. I prefer Skiplock's method of recognizing Wi-Fi networks, but the author has said he won't update the app for Lollipop.