I am a forum guy for nearly 10 years :) I don't like mailing lists this much. A good installed and maintained forum is more useful compared to mailing lists.
Some forum admins make a huge mistake by creating lots of forums/subforums when they just launch the forum site. If there are not much active users and a lot of forums then the users have difficulty "catching" the atmospher of the community and sometimes forum becomes even more and more inactive as time goes by. But this forum has just the right number of categories, I think. Even "General GTK+ Discussion" and "GTK+ Programming" may be merged for beginning. If someday the forum become more active with nearly 50 posts a day then we can begin creating new categories/subcategories. Such as creating new categories for each language binding.
I may suggest that changing the theme maybe, because lots of forums have this default theme for PhpBB and it became somewhat boring :) Something plain, elegant and reflects the GTK soul :)
For a really succesful PhpBB community you can look at OpenOffice.org Forums: http://www.oooforum.org
it is simply how a forum site should be.
And also YES, a project as big as GTK should definetely have a forum. Recently, there were discussions on GNUstep mailing list about opening an official forum page and they counted this forum among the examples while they are explaining why such a project should have forums. They also come to a conclusion that the project should provide actually an "official forum", which is very very right.