I don't get that. You say that C# provides a lot of nice features. Mono offers a C# compiler.
ObjC is for Apple only, because
- they more or less control the field and push it hard there
- other implementations lack the ecosystem
- other implementations are far behind
Mono gained quite a lot of traction (F-Spot, Banshee, Tomboy) on Linux, so far that it even allowed to 'backport' former Linux-only apps to Windows and OS X. You get the nice features C# provides, even if you are not on Windows. And you can rely on the same framework.
: Except when you don't: WPF is the biggest example, other areas might lag behind as well. But you _can_ write software for multiple systems _or_ focus on Mono (Gtk#, Mono.Posix/Mono.Unix) only to rapidly write software on Linux.