I've been using Rust to build a relatively large cross-platform command-line tool, and I've been genuinely impressed at how easy it is to get things to work on Windows. The Rust standard libraries Just Work on Windows, and they provide pathname handling, threads, environment variables, and a wealth of other tools. Many third party libraries will build for Windows without any problems as well.

I actually do think that Rust's type system helps a bit, because it forces me to consider weird portability issues (such as the way Windows permits slightly invalid Unicode, which Rust represents as OSString instead of String).

And of course, cross-compilation with MinGW is a real help. Sadly, OpenSSL still poses a fair number of portability issues, since it's notoriously difficult to link statically.