C - low level system programming plus to understand generic concepts about how memory is handled, stack, stack frame, heap and so on. These are helpful for understanding higher level languages
C++ - mainly std library (separation of generic algorithms and containers), templates, namespaces, but OO concepts as well. Templates meta programming will give you completely different perspective on writing software, this is compile time execution versus run time execution. Templates inheritance (static vs dynamic polymorphism).
Python - dynamic type system, list comprehension - functional programming (?), no memory management for developer, spaces for indentation
Objective-C - message dispatching (can dispatch to nil), dynamic type system (static as well), late binding, OO concepts