The Wii U controller features 3DS-like Circle Pad sliders instead of the tilting analogue sticks that have been the industry norm since Nintendo 64. That wasn't always the case, though.
An earlier version of the controller than the one we've seen included proper analogue sticks raised from the controller's surface, among other changes, as a NeoGAF comparison between the controller's pre- and post-E3 form shows. There are other changes too, mostly relocations of buttons and the stylus, but they're mostly aesthetic choices.
Nintendo has said the console and its new controller are both still in development, so the final pad we hold in our hands later this year could be totally different; we'll just have to wait and see.