The big ugly secret is that Microsoft web fonts were *not* well-hinted.
To get good web font rendering in windows microsoft had to workaround those web font bugs in the font rendering stack of windows of the time
That's why :
1. microsoft web fonts look ugly anywhere else (it's not the other font stacks which are bad it's the font themselves which have buggy hints)
2. other fonts (with clean hints) look bad on windows when they trigger web-font specific workarounds they don't need