The second most important improvement to Unity (in addition to the new UI) is an all singing, all dancing new event system.
Note
This section is just a UI overview of the new UnityEvent system; for a more code intensive overview, see Chapter 6, Working with the UI Source (it's all about the code, about that code, no tuples – a little Meghan Trainor reference for you, now that song that is likely stuck in your head as well!! 
).
At its core, it is not much more than a weak reference manager, marshalling calls between the input and raycasting systems and exposing these calls as events that any delegate or control can attach to. It's also much, much more, with its extensive extensibility framework.
Note
For more information on what Weak Referencing is and what it means to you, check out this great Code Project article on it: http://bit.ly/WeakReferencing.