Eh, I think using the stock market to gauge public perception and confidence in a company isn't very wise. Mainly because investors playing the stock market can be completely irrational (e.g., see how much Apple stock tanks after every hardware announcement), nor are they usually the target market for said products. That said, people still try to correlate things like this all the time.
It's probably unwise for investors to have knee jerk reactions like this though. I mean, this product isn't even coming out until 2012, and it's going to be a console that will be around for 5+ years. Nintendo (and all consumer electronics companies) are playing the long game and thinking long term. Your average stock market day trader doesn't.
I think investors are scared that the Wii U will be the Hangover 2 of game systems; that it will have everything the original has without enough originality to make it something unique and creative. While I myself am looking forward to the system and hope it catches on, all investors see is an HD Wii with an iPad-like controller you can walk around with. They might also see it as Nintendo playing a game of catch-up after they were out ahead of X-Box 360 and PS3 initially. The fact that Nintendo is forced to create a new system to be released possibly two years before X-Box and Playstation even announce their next system just to keep up is a sign that the Wii might have been a misstep that looked like a diamond. Again, this is from the eye of the theoretical investors.