While this may not end up being the greatest of reasons, a lot of my classmates shy away from .NET languages/MSFT technologies because they're more difficult to develop in Linux. Getting rid of that complaint would be super awesome.
You can use python or ruby.
DN and Java have a larger amount of classes accessible from any language, including Python and Ruby implementations.
Of course, as a developer, I hope there to be not just marketing and have a real initiative to bridge the divide.
With Swing you end up with a common denominator, which really sucks and you're better off deploying a web app with a local server and a stripped down Firefox.
Or you could do what the big boys do, share the business logic on all platforms, but implement the UI using native APIs.
Here's a sample: http://www.skybound.ca/ The OS X client is implemented with Mono + MonoMac (Cocoa bindings).
Please don't do this... we use a product that went this route at work and it's a pile of crap.