GTK needs to improve dramatically to catch up to the innovations of the toolkits in Windows and OSX.
This is highlighted with the simple "Hello World" test where you count the number of lines of code to create a simple Hello World. In GTK it is more than 10 lines or so and in other toolkits it is usually under 4 - 5 lines of code.
They also are abysmal in keeping bugs in check, I get noticeable bugs in GTK on Ubuntu 10.04 "Lucid Lynx". This is just unacceptable that a toolkit has bugs noticeable to the user.
I suggest the developers need to go back to the drawing board with GTK and do a complete rewrite to make it faster and more efficient. 2D performance is also poor with GTK so they need to address that as well.