I am thinking about cross-platform with nice programming language bindings (Java, Ruby and Python). What would be the \"flattest\" learning curve but yet enough powers to p
Gtk is a great cross-platform toolkit. Also, the bindings for ruby are all available in a rubygem, so its very easy to install. Gtk is used for many programs like nautilus, and has many, many capibilities. The tradeoff is that Gtk does so much, that its a bit complex.
When I write a gui, I refuse to hand-code every widget, I insist on a graphical environment to build my forms. I think that means using either Glade or QT Creator. I tried QT Creator, and found it to be so slow, that I couldn't use it, So I build my forms with glade.
When you build your forms in glade, the attributes of the form are saved in an XML file which can be read by your language. Many languages have "Gtk::Builder" modules that read the XML files and then use GTK to show the forms onscreen at runtime. So if you use glade, you can use the language of your choice (C java, ruby, python), and you don't need to "hand-code" all your forms.
Given the choice of languages, I'd choose ruby.
In fairness, I haven't tried wxRuby or Tk. But I know I don't want to hand-code GUIs. You can see my work on this at visualruby.net. I've used ruby with glade to create many GUIs.