Although I was preaching it for a while already that one should not use C today besides from teaching Assembly, it was only recently that one of the Gnome Developers brought up that topic too.
It is a fact that developing in C is more time consuming, since there is absolutely no support for any modern design patterns in the language, like iterator exceptions or even Objects/ Classes. Although you can help yourself on library level with things like glib, it will never achieve a level of usability like a language which was designed from ground up with those features in mind, like Python or even better; Groovy.
So now there is Vala which is a Java like language that compiles into GObject C code, but although it is much better than being forced to write plain C, it is still not what you can attract people with. With Vala you basically create a language for the Gnome ecosystem, which effectively means you still dont integrate people which did have anything to do with Gnome before. It also means that you cant use it for anything besides writing Gnome applications.
My advise would be to use some standardized managed language like Java/ D/ C#. So if one wants to contribute to gnome one has only to learn a new API and not a whole new language.
I mean thanks to Sun open sourcing Java, we have now a fast an powerful Virtual Machine/ JIT and its a fact that managed languages are the way to go. So why not take a jump and start using it instead. You would get tons of people already familiar with the language, since Java is currently tought almost everywhere. You also automatically get support for all the other languages running on the JVM like Python(Jython) or Groovy. That is without spending any effort on creating bindings and caring about main loop compability, since everything compiles in to the same JVM bytecode.