Neither of those is really an apples-to-apples comparison. QT is a widgets toolkit, Java is a programming language, and .NET is an application framework and interface between different applications and programming languages. Granted, there's some overlap between them, especially if you're bringing things like Jython in to give Java support for multiple languages.
I think the fact that KDE4 will also be available for Windows and MacOS X (also thanks to Qt4) speaks for itself. Qt4 has evolved quite beyond the realm of a "simple" widgets toolkit, by providing database, graphics and network backends, and by providing OS-specific integration features. Oh, Qt also ships with an interface designer that can integrate with many IDEs, including VisualStudio.
Hmm...lets not forget the fact that Qt's API's aren't subject to the whims of a megalomaniac corporation.
Because... it's a damn good platform? Because it actually solves some problems better than what's already available in the *nix world? I know it's hard for some MS-bashers to believe, but not everything MS does is a POS.
MS-bashers...well, its not like MS has given the FOSS community a reason to love them...
And aside from the fact that .NET is only an ECMA standard (I guess we can agree on how much those standards are worth..*cough* ooxml *cough*), Mono is basically relying on MS's good-will not to screw around too much with the APIs.
If MS were really committed to allowing cross-platform implementations of its technology, then they wouldn't have screwed Samba's developers by purposefully adding unnecessary complications to SMB2.
Listen to Leo Laporte's interview with Jeremy Alison which includes the SMB2 subject: http://www.twit.tv/floww14
(from minute 33, for convenience if you don't care much for the rest).
I guess we can call SMB2 a POS, or not?
What a classic example of what makes me hate Microsoft.
De Icaza says they can just continually create "double" implementations of the same API if they aren't backwards compatible...dunno, but how sustainable is that kind of development model....?
And for all the good points that .NET has: I don't believe they outweigh the potential problems its use may bring in the future.