Only everything important. Sorry to be flip but it is so hard to sum up. I knew absolutely nothing at age 24. I had no idea how to deal with people, how to manage relationships, my energy, or long term goals. How to balance and prioritize. I am nearing 40 and feel I am finally taking the first baby steps towards having even the slightest clue about what's going on. I'm pretty sure I will die before I get most of it straight. If you've mastered all the skills you need by 24 you've either set your goals on the wrong thing or you are a prophet. But that's just one man's humble opinion.

Either that or I never thought of the possibility that one could actually master these kinds of things to some extent.

I enthusiastically recommend "How to win friends and influence people" and "war of art"

I think this feeling is often eliminated by hindsight and change of circumstance. I've had the same sensation at a few points in my life until I broadened my fields of study, took on new roles, went broke etc.

In my own experience it's in the category of "You don't know what you don't know". If you are highly focussed on a narrow field of study or experience, yes, you definitely feel like you have "peaked" with respect to mastery. However, once you get exposed to "You now know what you don't know", a lot of things suddenly change.