The mild nihilism and the feeling that you're coming up to an age when the world is no longer all that interested in you.
That's not to sound dark and gloomy - quite the opposite - and I think that's what many of the younger crowd are missing here.
If you're over 40 and you want the world to pay attention, you need to offer some greater truth to the under 40s crowd.
I think the core idea that's resonating is that around 40 you're slowly starting to drift up out of the fray, and having previously been an the center of the universe, that's an interesting feeling.