Actually, it's not the goal of Mono to chase Microsoft.
Sure it lags behind in features, but they clearly said that they don't want to port any Windows-specific things (such as WPF), but rather the underlying platform.
It's not the goal to have all .NET applications for Windows run on Linux, but rather the ability to use this platform to develop for also Linux.

With Mono, you can use Gtk#, which is almost what Gtk+ is (and that is already present on the N900), so the charm of Mono is that I can use the same development environment with my favourite language to write code for my favourite mobile computer.

With .NET, this is it: I compile it once (doesn't matter it I do it with Visual Studio, MonoDevelop, csc, mcs, whatever), and it can run on every supported platform with the same executable.
Noone knows what is so good about it until he uses it for some time. Because of JIT compilation, the code automatically gets optimized for the processor it runs on, and you don't have to recompile it for every OS or architecture.

The other good point is object orientedness. (Although some people don't like it.) And you no longer have to care about managing the memory, nor the rigidness of the C++ languge, nor versions or dependency problems. The different versions of the same libraries can coexist without any problems.
(It is a SHAME that they couldn't do this with Qt.)