These days when almost every body is doing the agile thing, there is still one part of the software development process that is missing out - the design of the GUI.
Too often some snazzy web design studio is brought in, thumb rings and all, to produce mockups in Photoshop. These are then given to the programmers to implement to the last pixel.
I am sorry, but this is the waterfall process straight up. And it does not help that you implement it page by page or even component by component - that is just dividing the predefined work into smaller bits.
If you want to be truly agile, bring the usability people into the development process. Have them work iteratively as well in accordance to the stories being implemented, making paper prototypes the first round and refining in the coming iterations.
Now I can hear all the usability people go “But we need to see the whole picture”. That argument is what everybody has had when resisting the agile change. Remember, project leaders could not do agile development - how could they steer the project if they would not know where the project was going?
I have full faith in the usability community that they also can make the switch.