One of the problems that the Wii U faces right now is that it's not the cheapest option. I'm sure Nintendo would like it to be, but it's not. For less money, you can get an XBox 360 or PS3, and while they sure are old at this point, they still have tons of life left in them. We'll be seeing new releases on those consoles for many more years to come. In addition to that, you get blu-ray (ps3) or dvd (xbox) player, netflix, amazon, vudu, last.fm (xbox), hbo (xbox), Syfy (xbox), VEVO (xbox), Youtube, ESPN (xbox), Hulu, the list goes on. So what is the Wii U bringing to the table, for extra money and a smaller game selection, that will drive buyers to pick one up.
If you ask me, where Nintendo dropped the ball was in their Network/Internet multiplayer implementation. It was a CHORE to try and play a game with a friend on the Wii (what's your ump-teen digit numerical user id?). They had the elements in place including a cool blue glow on the console to let you know you received a message, but it was nothing compared to the Xbox Live / PSN service. I don't know enough about the Wii U to know how or if they've addressed it, but it's another hurdle they face since their competition is now entrenched with tens of millions of consoles out there with connected users accustomed to playing games with one another. You have to convince them that they, and their friends, need to buy one to continue their co-op and competitive gaming quests with one another, and I don't think Nintendo can pull that off.