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.
I credit Nat and Miguel for making Linux on the desktop a possibility. (I believe it was Ximian desktop on RedHat that made people finally stop and think, "Hey, there might be something to this after all.") That being said, I wish they had continued down that vein, instead of switching gears to try to make a Microsoft technology "cross platform," and basically failing at the ideal. I tried several times to run a Windows program under Mono, and never could get one to work. For all the talk about how C# is an "open standard," there was always some library that wasn't open-source and hadn't been compiled for Linux which would prevent the program from working.
I think you missed the point of Mono. Windows compatibility is one thing, but being able to use a really nice language that is C# and a really nice stdlib that comes with the CLR on non-Windows OS-es for development is something completely different.
The BCL is such a great tool that I am sometimes baffled by other languages heavy reliance on package managers like npm or gems to get basic functionality.
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.
I don't think that Mono gives you much more in terms of cross-platform compatibility out of the box than say qt or wx do.. It comes to planning, and practice. What it does give you is a really nice language (C#) and platform (Base Class Library) and more for software development. The platform makes it very easy to reference/utilize system libraries (even cross platform ones) to build apps faster on any targeted platform. Being able to utilize code cross platform is a bonus.
Also, in my world (Java) the stack trace is valuable as a software engineering tool. I don't know how folks develop in C, C++, ${other_non_scripting} day in and day out without access to that kind of information. Good debuggers and a ton of printf statements, I guess.