It seems that MSVC really has improved its support for C99, though, and I'm glad about that. What about C11? Does it support _Atomic/stdatomic.h? I can't tell from Googling, and I'm too tired to install the CTP tonight... If it does, it'd be ahead of OS X, hopefully not for long. The C11/C++11 memory model is powerful, way nicer to use than volatile plus OS-specific library functions, and, compared to other features, it's difficult to properly implement as a shim. It's too bad that only C++ can take advantage of it on some platforms.

Existing libraries are one thing, but when writing new code (and yes, I want to write new code in C), one of the big three compilers not supporting a feature is strong motivation not to use it... even if it would be really nice to have. Maybe in the coming years Clang will improve its Windows support enough that I can just tell everyone to swap it in for cl.exe, so I won't have to care about MSVC's standards support, but for now it and GCC are missing a lot of features on Windows.