> As previewed at Google I/O, Lollipop is our largest, most ambitious release on Android with over 5,000 new APIs for developers. As a developer not initiated to the Android platform, the second half of this sentence is a very scary thing to read.
Some things have been completely rewritten :
Notifications have been using the same expanding API since version 1, it has been strongly revamped. The RemoteControlClient (which manages the lockscreen and is used for some other things like communication with Chromecast) was one of my least favorite parts of the Notification code, it is being phased out and replaced by a new, simpler (and hopefully more reliable) one.
The ActionBar API (the bar of controls at the top of every android app) was initially based on the old menu API (from the time when we had menu buttons on Android). It was a very nice move to soften the menu button > actionbar transition but it is now adding unnecessary friction. Since the ActionBar has been redesigned, it was a very good opportunity to deprecate its API and replace it by a completely new one.
ListView (the widget that handles lists) is one of the oldest Android widgets. It has been conceived in an era where displaying a static list was all you wanted. It is a central piece of almost every app UI and it is comically inadequate. It is going to be replaced by RecyclerView, which take into account modern requirements on lists (adding, removing items, handling gestures, ...).
Many other changes are just additions. A better animation system, a tinting system for on screen elements, some new widgets, ... There are also some completely new APIs but many of these will only be used by a handful of developers (it is not every day that you need to convert the content of your screen to a PDF and print it).