I started programming for iOS via the same route, playing around with Objective-C in Xcode, but have since learned C. I have found that an understanding of C proves to be essential if I try to do anything reasonably complex (e.g. https://github.com/conradev/BlockTypeDescription).

I have also found that another area in which newer developers fall short is understanding how Xcode works. Those familiar with interpreted languages do not have to compile their code. While they may understand the concept of 'modules', they don't know the difference between static and dynamic libraries, what 'linking' is, and how the compiler finds header files. Someone not knowledgeable of how this works will come upon innumerable headaches down the road, especially trying to integrate third party libraries.

I would suggest a supplementary post entitled "Learn How Xcode Works, You Cheater"