Java desktop applications using the Swing toolkit are notorious for non-native look and feel compared to native OS X apps using Cocoa or even Carbon. To a lesser extent this problem affects Java apps on Windows, and all other cross-platform GUI libraries, from Qt to Tk.
Does the newly announced .NET port to OS X take any new steps to avoid this situation and get closer to the goal of "write once, run anywhere" with native look and feel? Or will OS X .NET apps have a non-native "Windows-y" feeling to them?