>>3
Lower-level CS concepts: algorithms, data structures, etc, and their applications. Lower-level languages that can be compiled, e.x. the C-family, Lisp-family. Maybe also historical languages, machine code, and assembly. Programming techniques: refactoring, planning, testing, etc. Linux. Maybe also the internet protocol and its layers.
>>5
Interesting. I'll have to skim through.
>>6
Mastering every layer of abstraction to master the machine. The languages I've been exposed to have such a high level of abstraction that I think I've missed understanding some underlying CS concepts and techniques. It's a real challenge to migrate from Python to lower level languages since you are used to having almost everything managed for you at the cost of fine-grained control.