My biggest complaint was the use of digital triggers, its criminal. Also wish the screen was sharper, it always felt a little blurry and washed out.

I just got one like 2 weeks ago. Only complains are the low res screen, small battery life (then again I mostly use the Pro so not really an issue, yet) and just the way you have to hold it (can make my hand hurt after a while).

It's has no problem. According to Nintendo guys (2012-2014) it "differentiates" (lol) from the competition. It's Wii Us unique selling feature.

Rant aside; it's of no use. Nintendo never put thought before integrating it to the system.

My biggest problem is the battery, because I do like the layout

i think most peoples problem is the price

My main problems with it are the horrendous battery life, the low-res resistive screen, the size and no analog triggers. I like the Gamepad, but it could've been better =P

Pros: It's great for Mario Maker
Cons: It's really only great for Mario Maker

I didn't like the button layout and having to look at the screen on the controller.

The main issue is it is of no use to many genres.
For fast paced games you do not have the time to look at the Gamepad. It has no good use in platformers, fighting games and action games, and most games on the Wii U were from those genres.
In genres like racing games it could be useful as a back mirror, but the Wii U isn't powerful enough so the graphics have to be scaled down to support that.
The genres it is useful for are games with heavy inventory management, open world games, RPGs and the likes, and there were too few of those on the Wii U.
It is easier and faster to touch a screen to manage your inventory (Deus Ex, Zelda) or fast travel to your destination (Xenoblade X, Splatoon) than to select the destination per menu. You can draw or drag and drop things around (Miiverse, Mario Maker) You can also type quicker with an on screen keyboard than with a standard controller (apps, scribblenauts, "enter your name").

In short:
The biggest issue with the controller is, there weren't enough games that benefit from a touch screen.

I also believe the people that hate the controller mainly play games from the genres that don't benefit from it, while those that like it enjoy those genres that benefit from it.

I agree.

Before Wii U release, I saw many uses for Game Pad, then, nothing came out.

In some cases, like Star Fox, when Nintendo decided to use GamePad, they ruined the game. In Splatoon, we could be playing local co-op if GamePad didn't exist. GamePad is there only for a map and to jump.

The best Game Pad use: Hyrule Warriors. One of the best local co-op experiences I've had.