Seems like 99% improvements are made to the editor. The only new programming feature I found is "Create objects using a string of their name instead of having to pick a specific object".
It's a shame really, I can think of many new events and expressions that could be really handy, to name a few:
Refer to instance variables by their string name
Reset global variables from a selected group only
Reset static local variables
AngleDiffSigned expression which returns negative number if angle2 is counter-clockwise from angle1
object.isVisible, isFlipped, isMirrored expressions
Actions to change text settings, like text alignment, text wrapping.
Also, there are many highly popular plugins for C2, I don't understand why not implement some of them as standard features and behaviors in C3?