C is the sweet spot where I can extend my programs to do high-level stuff while still keep my hands down on the actual hardware I'm programming. I like that a lot, probably because I grew up banging hardware. Or maybe it is because I like to keep in touch with the actual device that I program: bending some definitions, C is what my machine does. Even if I embed Lua and script parts of my program, I'm still conceptually working on a C runtime, complete with addresses, pointers, integers and registers. That's why I also like LLVM as it abstracts away different instruction sets into a generic high-level instruction set, much like C abstracts different assembly languages into a generic high-level assembly language.
C also has the property of being most enjoyable code to read. I've spent a lot of time just reading C out of sheer enjoyment. C is tricky: it can be a total mess or it can be terse and beautiful and clear. No matter what, it describes exactly what it does to my machine. Read some of D.J. Bernsteins source trees to get an idea of how neat C can be.