Secondly, and that's even more of a reason, it's not very powerful. While Windows programmer might be used to it - unlike both Linux desktops GUI toolkits - it does fix widgets with coordinates on the form. GTK#/Qt# use an intelligent boxing-system. The user can resize the form - it will be never to small. Internationalization is much easier, that way, because if the text doesn't fit in the room, the box is automatically made bigger.
The way Java did take is called Swing. Swing looks consistent (ugly) across all platforms. And it is slow. Very slow. In fact that's one of the reasons why people think Java is slow in general. But we have better options. There are two GUI toolkits out there, one used by KDE desktop, one by Gnome. Both are ported to Windows and MacOS X. Gtk, the Gnome toolkit, is even ported to Framebuffer and BeOS. Gtk# (the C# binding to Gtk) is very fast, currently Qt# is even faster. Much faster than Swing.
So, what GUI to take? My personal favorite is GTK#. Why? The Windows and MacOS X versions of Qt are not free, currently. The developers are working on porting the X11 code to Windows. But currently it is not done. Besides that I'm a Gnome fan, and more used to the Gtk+ toolkit.
The Mono Handbook has a big section GNOME.NET. Not all of Gnome is multi platform. But we have clearly marked, what is. The portable part is: