Not really buying it. First of all, you're just comparing desktop Windows with embedded versions of Linux and Mach/XNU. Secondly, the latter two systems are not exactly free of cruft and legacy either.
You know, I let it slide when the comment you replied to said it, but I disagree with this one. IMO most people who criticize Win32 just aren't familiar with it. I've heard plenty of people dismiss objc on similar grounds: simple unfamiliarity becomes "this is bad". Once you get over the initial learning curve it's quite easy to be productive with it.
Kernel32 user32, advapi32, etc. Are you really going to tell me that the namespacing and naming convention for their API, which mind you, used handles and annoying type defs, was anything spectacular. I really hope you are kidding.
I reverse engineered windows for years, know a huge amount of documented and undocumented api from ntdll and so on. The API is water trash, you lose when you question my know-how on this topic.
I was on the Windows team for a few years. I have seen the source code. There is cruft as there will be with any project of its age, but it's not nearly as bad as people say, especially if you can navigate it well. There are some good ideas underneath. COM is generally a good idea. The mental model of message pumps and window procs doesn't really have deep flaws, and lots of UI frameworks on other platforms have the same ideas under the hood. You mention ntdll, it's a shame more of that isn't public, because those APIs are often cleaner than their public equivalents.
The API, mainly. Compare the size and cruftiness of the Windows API with the Android or iOS equivalents that a developer programs against.