There are three things to me that make any Nintendo system worthwhile.
I've owned every Nintendo console system and a few of their handhelds.
1) Nintendo games - This is the first thing, one that you can't take away. Nintendo makes great games, ones that you can't get from other developers.
2) Games (not from Nintendo) for the kid at heart. I love it when third parties release something for Nintendo platforms that might seem a bit risky it were released for the edgier/more powerful consoles. Sadly, developer confidence is a factor in whether these games even get produced, and the companies making them are more risk-averse than ever.
3) Great controllers - Nintendo has nearly always set the standard for terrific control schemes. Analog sticks, controllers that are comfortable to hold, unique ways of interfacing with games. Unfortunately, this time they've sort of concocted a mix of several different ideas that they've used in the past (touch screen, motion controls, different screens for different players) into one pot. And judging by the reviews so far, most of the chefs don't know how to read the recipe.
The thing I love the most about Nintendo systems is that they've always been a haven for single player games. Yes, there are great multiplayer Nintendo games, but I can always look forward to a single player experience.
So while I wouldn't necessarily write off the Wii U, I will say that for the first time, I'm not sure what Nintendo hopes to accomplish with its new system.