The most important GTK 3.4 improvements are the support for most of the properties of the CSS3 backgrounds and borders family, linear gradients and the long requested inactive windows theming feature, which is showcased in GNOME 3.4.
What is coming round the corner is the implementation of support for transitions and CSS animations that will make GNOME 3.6 look cooler than ever!
Another feature that will be in GTK 3.6 is support for multiple layers of background images for a single element, as specified by CSS3; in other words, where you could only render a gradient or a solid color before, you can now render an unbounded set of images and patterns. If you combine this with the other CSS3 background properties, and transitions, the possibilities of what you can do are basically endless!
The tools of creation are equally important with the imagination and the creativity of the developers, and it is great to see that the tools are getting more and more improved and enriched giving GNOME developers more power and less limitations.