I would love to be able to support Linux and Mac but am not willing to dig into C++ / Qt or similar alternatives. It's a hobby project, which I should have mentioned previously, not a commercial work.

C# is great and WPF/XAML is OK; I can't really make any profound comments as I've only done web development previously and am not familiar with any other desktop technologies.

With regards to HTML5/Javascript superseding WPF in the future, I am not convinced - Microsoft should shed some light on their plans in the near future (the Build conference is in April I think) so we'll see.