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I am not mister de Icaza but or Larry Ewing (creator of F-spot), but I will take a stab at this. F-Spot and other great Gnome# applications that are making it big in Linux now days rely a lot on underlying technologies that are platform specific to GNOME and even lower level pieces that only exist in *NIX. We have just began to make parts of Gnome# available on Win32.
Gtk# the UI library framework written in C# that makes writing GTK GUI applications with ease, makes extensive use of a feature of .NET/Mono called Platform Invocation (PInvoke). This means that although when you are writing your applications you are doing it using C# semantics, at runtime there are plenty of calls to unmanaged function calls that may not exist yet on Windows.
In time we will have full parity or very close to it (97% is good enough?) with Windows, Mac OS X and others, but until then the folks taking full advantage of mono/Gtk#/Gnome# are taking advantage of Linux. Should you?