I'm not sure if this was mentioned before but the eye's take a little bit of time to refocus when you change your eye's focus point. While it is great exercise for the eye's and can stop the degeneration of the eye muscles, it doesn't lend itself well to gaming machine that requires fast reactions much of the time.

My belief is that Nintendo made the Wii U Gamepad out of health concerns for people's eyes much like the Wiimote was to get people moving/exercising while playing games.

Just my opinion but I hope Nintendo developed the NX with a different mind set.

Interesting...

My main problem with it is the battery. If it shipped with a 8+ hour battery, would have liked it a lot more. Also better triggers

IMO only problem is weak battery.

Problem checklist:

Short battery life

Size

Shape, both from an aesthetic and comfort perspective.

If they had taken a year to actually evaluate the Gamepad's feature list, they could have slimmed it down alot. The speakers are bigger than they need to be given there is a headphone jack, the NFC chip could go under the screen, the screen could honestly be the size of the 3DSXL or standard N3DS screens, the front facing camera is entirely pointless, and the home, tvii, +, -, and power buttons could have been better located, Remove and alter those things and you could keep all the actual important gameplay features of the Gamepad (the touch screen and gyroscope) while making the controller smaller, more ergonomic, more aesthetically pleasing, lighter, give it a better battery life, and make the thing cheaper to manufacture.

Which is why the Gamepad to me screams "prototype." Which is not surprising since the Wii U was clearly a rushed product.

There is a couple of games where the Upad is preferred, but I usually use the pro controller if given the option.

My main problem is that it's just plain terrible, I find it so annoying! I leave it sitting on the charging dock and use the game cube controllers.

When I play ssb with my friends everyone races for the GC controllers, no one wants to use the tablet.

I have to agree with the eye refocus thing. I get around it by pushing my glasses a bit down my nose and look over the top of them (I'm long sighted) .

When I first got the Wii U it took me an hour or so to adjust to the idea of when to look at the Gamepad and when to look at the TV. It's not the same as glancing between the screens on a 3DS. It's also less ergonomic to use as a standard controller than the Pro.

I also wish it had a longer battery life.

However, for touchscreen games it's brilliant, and the stand comes in handy for offscreen play if you still want to use the Pro with it.

And it's the first TV remote we have never lost.

For me the game pads weakest trait was ergonomics. Aesthetically it was great for me btw. I just can't grip and touch it comfortably. I bought the battery pak. Too unbearable with out one.

The main issue is the display itself.
Gamers come to "know" their controller so well they dont ever need to look down at it.
Their focus is on the big screen, the TV, and they are absorbed in the game.

Haveing to constantly shift focus, and looking down on the controllers display is just not a good concept.

Get one of these (wii U pro controller):

See, I agree for many cases that off-tv is the only significant advantage.  However, having played Xenoblade Chronicles X, Wind Waker HD, and Twilight Princess HD, I will say that the convenience of speedtravel (in XCX) and inventory management (in the Zeldas) is very very nice and one of the things I sorely miss in Dragon Age Inquisition.  Though that also comes down to the UI in Inquisition being just plain bad. 

The other thing I miss when playing Inquisition is ease of traversal but that's another rant for another time :P