I think Ive said it before, but there was a point in my game development were one small feature in C3 literally enabled me to continue game development that in C2 would have been near impossible or at least frustrating enough for me to quit.
That was the point in my project where the number of events became bigger than my ability to remember what those events were doing and where they were.
So the C3 game changer for me is the humble search/find functionality.
The ability to instantly see all the events that use a certain variable, or sprite , or function call etc, even if they are spread across multiple event sheets, is the most useful new tool that C3 offers IMO. Everything else is icing on the cake.