The very small list of unsupported APIs are extraneous and unnecessary in the context of cross-platform code. So, what APIs are you referring to, specifically?

http://www.mono-project.com/Compatibility

The big ones are: WCF, WPF, WWF... Guess what the first W in all of those acronyms stand for? Windows.

EDIT: It's right there at the top of the page - "Everything in .NET 4.0 except WPF, WWF, and with limited WCF." - I don't see a problem.