alt Hacker NewsA couple of examples what's missing from C from a low level perspective: Multiple return values (why can't I have a function using more registers than rax or the stack to push back more values?), sane varargs, stack manipulation (an enabler for coroutines and a lot more)
Generally, in an ideal low-level language I wouldn't expect to be hampered because the language is designed for a notional abstract machine that doesn't have things like an actual stack and contiguous RAM.
Also C isn't well suited to a modern multi-core computer. The "fix" in C11 is to just say we have the C++ 11 Memory Ordering Model and here are some built-ins for the ordered atomic operations that implies.
Having any model at all is probably most of the value and only one part of the model (consume ordering) is unimplementable fantasy so it could have been much worse, but it's hardly a triumph for a "fundamental" language is it?