Mono lets you run some .NET code on Linux/Mac OS X. Some of my favourite pieces of Microsoft technology are missing or too broken to use - for example WPF is an excellent tool for building desktop applications but it doesn't work on Mono, F# sort of works until you try doing recursion with abstract data types.
IMHO it's kind of silly to expect things like WPF to run on multiple platforms - and I'm actually glad it does not.
The world doesn't need more apps with lowest-common-denominator UI. Ugly, kludgy, doesn't work well on any of its platforms because it cannot leverage any platform-specific UI strengths. If anything, the last decade of desktop and mobile computing has shown beyond a doubt that a highly refined user experience is paramount for anything that faces general users. In fact, an app with fewer features, but a superb user experience, will trounce a more capable app with terrible UI (see: Apple).
This is also why I dislike apps that use the UI components from Java - they don't look, feel, or work like native UI widgets. Their layout is often confusing, since they are trying to appease the learned design patterns of multiple disparate OSes with a single design. They end up being awkward, frustrating, and a very poor experience. I have never met a Java-UI app that even reached an acceptable level of usability.
For me, having Mono run well with the obvious cross-platform bits (threading? file system access? etc etc) is enough. I for one really like C#, and would much rather write it than Java.
Eh, I'm going to disagree with you there. Qt on OS X still produces dramatically non-standard looking UIs.
I don't think papering over the significant differences in UI paradigms between Windows and OS X is something that is fundamentally solvable by a widget toolkit. You can make a toolbar full of buttons work on both platforms, but you can't solve the fact that cramming 40 buttons onto a toolbar feels completely non-native under OS X.
