I don't get the appeal of gamepad-only play. Why do I want to play a game on Nintendo's inferior resistive-touch 'tablet' when I've got a Retina iPad. But I've not used it, so it might be the bees knees when everyone's competing to for the one TV that has all the toys in the house.
First off, you don't have the (extremely common) problem of someone else wanting to use the TV to watch a movie, TV show, football game, etc. so this one aspect of the Wii U might not apply. Second, I was talking with a coworker today about how he was playing Mario 3D World and he ended up looking solely at the GamePad even though the game was also on the big HDTV. It's amazing how preferable the experience is. Third, until an iPad has a relatively steady flow of Nintendo-quality games and a control scheme that isn't awful, there are plenty of reasons to play a game on the Wii U GamePad.
What can Nintendo do? Reading Matt's and Morgan's rebuttal to my thoughts, and thinking on my ambivalence to the WiiU, what I find lacking is substantially marketing.
My perception of the WiiU is based on the Wii: I didn't like its dodgy, erratic motion control (I always felt like I had palsy, holding the controller and seeing the digital hand shake about on screen). Lacking recent experience, I expect the WiiU to have that weird, unstable feeling. I have negative expectations about its fundamental control scheme. And then they have this odd, unresponsive gamepad. I've fiddled with one at Target of BestBuy: in contrast to modern capacitive screens, the resistive screen felt unresponsive and coarse.
The game system also seems like a secondary system. I buy it after I've got a main system and played all its games. The N64 and Gamecube, I recall having top-notch third party games to rival other systems. Jet Force Gemini. Wetrix. Rogue Leader. Turok. I spent more time playing non-Nintendo games.
But with the WiiU, I see it as the second choice. It won't have the mainstream games (Bioshock). it would be like buying a PS3 if the only games it played were the cool PS3-only games games like Journey and Last of Us.
From Nintendo, I want an Apple-store like experience -- a quality in-store demo. I need to play games on the console that reveal why I want to own it. And particularly, why I'd buy it over a One, PS4, or even used PS3 or new iPad Air. Because for me that's what it competes for in dollars and attention.
Totally agree on Wii U's marketing. Nintendo has dropped the ball for the last year and in a bad way, which is sad since the Wii had amazing marketing.
As for the controls on the Wii...how many Wii games did you play and what were they? I can probably recommend a dozen or so games you might not have even heard of that are amazing, whether they use the Wii remote's unique features or not. THAT is why I love Nintendo systems and their goofy control schemes. And how do you feel the GamePad is unresponsive? I've played multiple games of different genres and been very satisfied. Arguably, the best proof-of-concept for it is Rayman Legends, which uses every feature to make an amazing experience even better than on the other consoles.
I should point out that a lot of the N64/GameCube games you mentioned were made by companies Nintendo partially owned or that Nintendo paid for. So while they weren't Mario or Zelda, they were still "Nintendo" games. And those types of games (along with exclusive-to-Nintendo-console games) are on the Wii and Wii U, as well. It DOES have mainstream games (Assassin's Creed, Call of Duty, Watch Dogs, etc.) but just not ALL of them. Nintendo (and Sony and MS) have stated multiple times that they're not really competing with the other consoles in the same way. I absolutely see a Nintendo system as a system that I would own in addition to a PlayStation or Xbox, but that's because I really like games and want the chance to play as many as possible.
Honestly, I cannot understand why you would compare a gaming console to an iPad. The iPad does a LOT more things but doesn't really do what a console can do in as good of a way. Heck, an iPad can only dream of doing what a 3DS or PS Vita do. If it comes down to only having so much money to spend on tech, then its a different proposition of "entertainment/features" than "game experience." An iPad is not a gaming machine and the Wii U is not a portable laptop replacement.
I may not fully understand the point of the Gamepad, or perhaps I'm not be it's target audience. It seems to me that the only thing the Gamepad gives me as a benefit is that I have a map or inventory management screen in my lap, or I can play games not on my TV. Why would I ever choose to shut my dedicated home theater room off and play the same game on a micro screen? I can possibly see this for familys, but I'm a single individual, and have nobody trying to kick me off the TV so they can watch "The Bachelorette".
I admit that I did like some of the features given to me by ZombieU, I thought the use of the pad was pretty decent in that, and was hoping to see more of that, but as of right now, I haven't seen anything since then make good use of the Gamepad. In Super Mario 3D World, the use of the gamepad is more annoying, and tends to just give me minor whiplash on the few stages that require it looking down, looking up, looking down, looking up. I really just want to put it away.
That said, I want to emphasize that I like this system, and I really am looking forward to new stuff coming out for it.
The GamePad is, basically, an additional screen. Yes, a lot of games use it for minimaps and inventory but that is really up to the game developers to utilize in interesting and new ways. It sucks that they haven't but, personally, the off TV play is worth it alone for me. Yeah, I lose a big TV and surround sound but....well....I don't really mind. If something NEEDS to be played on the HT then I'll do it but a lot of games don't scream for it. There are other features of the GamePad (like TVii) that are way better by not being attached to the TV, but your mileage on those will vary a lot.
In general, Nintendo makes hardware to support their games. Regardless of the control scheme, you buy a Nintendo console to have the privilege of playing their stuff. If you like Nintendo's games enough then you will buy and love their hardware. They have never let me down.