It seems to me that it DOES pretty trivially give you polymorphism; just change the function pointers in a struct and it behaves differently, while retaining the same type! Inheritance can be implemented, too, by copying function pointers and other struct contents around.

If you play with languages that use prototype-based OO like Lua and JavaScript, you can see pretty clearly how to implement such things. Implementing your own OO system in Lua is an experience I'd recommend for anyone who uses OO; it's easy, and you'll almost certainly learn something you can apply elsewhere.