First of all, adoption of a multi-core CPU for the first time. By having multiple CPU cores in a single LSI chip, data can be processed between the CPU cores and with the high-density on-chip memory much better, and can now be done very efficiently with low power consumption.
Nintendo also published a few images of the Wii U’s motherboard, which you can see at the top of this post. The engineers also discuss the fact that HDTV has become the standard in Japan and much of the world, which is why Nintendo made the Wii U with high definition standards in mind. Perhaps that’s a generation too late, but there’s no doubt that Nintendo put some serious engineering into the Wii U guts.
Read the whole interview with Nintendo engineers here.