I already told my colleagues. They were going to find out soon enough anyway, so I figured I might as well give them the heads up to help them plan ahead at how they are going to replace me. I haven't officially handed in my notice yet though; and I reserve the right to hold off on that until everything is in place.
I can particularly relate to your points 3 and 4
naturals. Number 3 because nobody is perfect. Everyone has strengths and weaknesses, and - while it's possible that some people performed amazingly well at everything - I haven't personally spoken to one successful candidate who did. And number 4; because without having invested a huge amount of time preparing I would not have made it.