But I think they're of a piece. All of these things are "meatspace" problems. We are constantly fighting the temptation to try to intellectualize the whole process of making software because the center of the problem is trying to interact with a machine. Some don't fight that temptation at all.
You see this problem when people fight automation because they expect everyone to execute a manual process perfectly every time, even when the consequences of failure are very high. They insist that everybody white-knuckle everything and think less of you if you don't agree. They victim-blame. When they're the founding or oldest surviving members of a project, you're gonna have a bad time. People who could fix the problem move on to other jobs.
Naming things is really the same class of problem. A bad name calls to mind assumptions that are not relevant, and you are in constant danger of making category errors every time you think about that part of the code. A bad name becomes a sort of attractive nuisance.
The C problem is a matter of motivating people to do the right thing. Because it's difficult to spot, it's an invisible sin. It rarely comes up in code reviews, so nobody will call you out on it, nobody will make you fix it, finding it after the fact is daunting, and the only people who will care (eg sales) are so often the source of other problems that pleasing them is almost a character flaw. If performance is a feature, then constant time regressions are one of our worst games of whack-a-mole (behind security and internationalization, maybe ahead of sane logging).
[ETA:] I'm just a weirdo who likes to tilt at some kinds of problems I find hard, instead of dumping them on someone else.
Judging by those, the most important hard parts are around planning and estimating work, and correctly understanding when those make no sense, and in validating code for correctness, especially over time.
If we ignore importance and just go with what's the hardest single task: naming things.