Besides his skills as a software developer, I am impressed with the fact that he has held on to his vision and led his company from Ximian, through Novell, then Attachmate and onto Xamarin.
It is consistent, well behaved and while not complete, it is very solid. The only complaint I have are that it has some dark corners (System.Diagnostics come to mind) and the fact that non of the classes are easily used for testing and that I have to wrap a lot of basic functionality like file system access or the TcpClient to be avle to test my classes.