This is the most important thing for me. There are things which I am currently incapable of binding properly in winapi due to a lack of FFI features. Things like unions, #[repr(align(N))], #[repr(packed(N))], bitfields, COM calling conventions, dlimport, static-nobundle, unsized types with thin pointers, and more. In fact, I don't think Rust has had a single FFI feature implemented since 1.0 (the most useful thing for me that did happen since 1.0 was making glob imports actually work). Several of those features I listed even have accepted RFCs, so I have hope, but seeing them take so long to be implemented, nevermind stabilized, is really demoralizing.
This is also hugely important, not just for me but for everyone. I'm going to some fairly extreme lengths in winapi just to minimize the amount of code that has to be compiled so compile times can be fast: getting rid of almost all trait impls, making enums simple integer constants, and abusing cargo features. I only have about 10% of the code currently compiling in winapi 0.3 but it already takes 4 seconds to build. So, when I do eventually get everything updated, should I look forward to 40 second compile times?