I am looking for a 2d open source GUI engine on Linux, to eliminate our reliance on Windows. Our current application is written in C# + WPF, which is incredibly slow and costly. It is stopping us to do anything awesome. If there is anything in Linux that can offer what WPF could offer (easy to use designer, animations, layout and component controls, etc) I would rather rewrite my entire code base in C++/Java in Linux.
Qt is one option but I am a little bit worried about its support in the future, given recently Nokia fired the entire team.
I would still suggest Qt. Even if Nokia destroys all of their support for it, Qt is under the LGPL. That means that the source will still be available and free (libre), and the KDE community will keep it going. They already are to a degree anyway. Also, although Qt is still C++, it is a very modified extension of C++ through the meta object compiler (moc), a special sort of preproccessor. I think you won't regret the choice to use Qt.
Of course, if you want to stay on C# and .NET, there is the open source Mono initiative, who are trying to create a drop in replacement for .NET and C# that is mostly compatible. I don't know if they support anything besides Win Forms, but there are bindings for almost every graphics lib out there, so you can use GTK (a binding called GTK#), Qt, WxWidgets, and a lot of other things.
Also, there are GNOME's Glib and GTK+, which are the other major interface libraries for Linux. They are C oriented, so you might have a harder time adapting your code though.