Windows and OS X use different methods of hinting where Windows attempts to stay true to the typeface and OS X attempts to provide a provide a smoother, more readable version. Webkit browsers give you some level of control with subpixel rendering CSS attributes, but fonts might still look awful in Firefox or IE on Windows.
Also Windows 8 uses subpixel positioned grayscale, rather than subpixel positioned ClearType.
When you rotate a tablet by 90degrees ClearType doesn't work.
