As per a post on the company ZenTalk forums, the Android 5.0 Lollipop roll-out will begin in April 2015 and continue until June 2015. Thanks to the popularity, Zenfone-series will be first to get the Lollipop update in April 2015, followed by Padfone S and Padfone Infinity A86.
For most of the Asus phones set to get Android 5.0, it will be the second major Android update after KitKat. There is no word on the official change-log as it is likely that Asus will be making a number of changes on the Zen UI front, which are unclear right now. On the OS level, Android 5.0 should bring enhanced lock-screen notifications, ART as default runtime, UI enhancements, battery improvements, and more.