Summary for the lazy?
1. Latency was a challenge from the beginning.
2. All processing is done by the console. Even when you use the camera, it's sent to the Wii U then to the GamePad to be displayed on the screen. The camera was almost cut, but they got it to work. Video from the is 30 frames per second. If you think that's low, it was originally 5 frames per second.
3. Models of the GamePad were made with a 3D printer then fine-tuned by hand.
4. Software for controlling the GamePad was developed at Nintendo Technology Department in the US; the design was done in Japan.
5. There's some talk about how they reduced latency which I don't feel I understand enough to confidently explain. It
sounds like compressing/decompressing is traditionally done per frame, but this would cause delays. Instead, the Wii U breaks down that image into smaller pieces which are then compressed/decompressed before being sent to the GamePad.
6. There were latency concerns with how the GamePad is held and who or what is in the way, but they worked it out. A metal TV stand, for example, may affect the radio waves and weaken the signal between the console and GamePad.
7. Some images are more difficult to send wirelessly and display than others like a moving grid. They found this early in development and spent a considerable amount of time making it work.
8. Wireless communication technology in the Wii U was co-developed by Broadcom.
9. The design changes between E3 2011 and E3 2012 were apparently from internal pressure, notably Miyamoto and Iwata himself. No mention of 3rd parties urging changes to the circle pads, for example. The grips were an important addition as the flat original GamePad was apparently uncomfortable to hold for long periods of time.
10. Weight was whittled down by the gram.
11. Iwata announced the near field communication capabilities before the team finished the software to make it works. It seemed like they weren't sure NFC was absolutely going to make it in.