In general some things are of greater use to library developers than programmers, but it helps to have an idea about things even if you don't know them in great detail.
I think the number one thing fitting that description is templates.
Even if you don't understand how to define templates, you need to at least know how to use them and the basics of how they work to be able to use the stdlib effectively.
I don't cover templates in depth in this document since they predate C++11 (though the hardest item on the list is a new feature of templates), but I may consider giving an overview of them elsewhere if there's interest in doing so.
I do cover some of the use cases, but I think there are probably only a few things that need discussion about when to use them (like scoped enums) since a number of features are self explanatory (like
static_assert and range-based for loops).
It's hard to say at this stage which topics are more likely to be used.
Normally virtual functions and dynamic memory allocation are avoided in embdedded systems, but I suspect it's only a matter of time before someone tries to use those, in which case some of the harder topics will be important.
I can honestly say there's only three features on the list that I've never used. Admittedly there are one or two of the more advanced topics I think would be unlikely to be used (especially the most advanced topic).
Overall I think all of the 'easy' things and most of the 'medium' things are crucial, and depending on whether people are using virtual functions or dynamically allocated memory, two of the 'hard' topics and one of the 'very hard' topics could be pretty important.
Hrm, I think the stdlib deserves its own tutorial if the documentation at en.cppreference.com is not enough on its own.