Last night I downloaded the Qt 4 preview and had a play with it, so far I'm very happy. The new painter code seems great and the GL painting demo suggests a lot fun ideas.
Qt 4 solves one of my big annoyances with the Qt API by introducing a common base class for items in list views, tree views etc. This change will make it much easier to let the user choose how information is displayed as we can have dialogs that switch between iconviews, listviews and trees without having to write the same code three times. In fact, when combined with the new model-view facilities (which I have yet to examine) this may be something that can be encapsulated in a single widget.