I think there are a few important points to discuss here:
1. Method used to take screenshots of Wii U games
2. Wii (and GameCube) derived architecture of Wii U
3. Art style pursued for Wii U games released thus far
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1. It's already established that screenshots posted to Miiverse are compressed in size, reducing the graphical quality. They're good enough to work in the context of Miiverse as part of a social network that people can "Yeah" or comment on, but they're not good enough to base an objective opinion on the quality of the graphics the Wii U can deliver. An HDMI capturing device is a minimum requirement to take a raw 1920x1080 screenshot suitable for a discussion like that. Even then, it is important to make note of the settings and capabilities of the capture device to remove any bias (e.g. exaggerated sharpness or contrast setting).
2. Wii U uses an architecture very similar to the Wii, which in itself is similar to the GameCube. It's why backwards compatibility is achieved in hardware without extra components and it's also the reason why developers have started to push the Wii U to its limits early on. As time progresses and developers become more familiar with the Wii U, graphical quality may also increase noticeably. However, the Wii U is essentially running on the same architecture as its two predecessors, meaning that from a developer's perspective, the hardware requires little exploration as it's already a familiar platform to develop for. This is not necessarily a bad thing, as a completely new architecture takes time and care to develop graphically detailed games for. Take a look at early PS3 games and compare them to current PS3 games to see just how much getting to know the hardware better enables developers to make games with better graphics. Wii U however is already at a fast pace to hit its graphical limits, as developers already know their way around the hardware thanks to the Wii and GameCube before it.
3. As with any discussion about graphics in games, art style or visual style is an important part of the discussion. Taking a game like Zelda: Wind Waker HD in the context of measuring the graphical capabilities of the Wii U is bad practice. A game like Pikmin 3 or NFS: Most Wanted U, for which the intended art style is realistic, makes for a much better choice in this discussion. There's already enough accounts to state that the Wii U is graphically more capable than the PS3 and Xbox 360, but falls short in comparison to the PS4 and Xbox One. Considering that a recently released flagship title like Mario Kart 8 is actually a 59fps game with no anti-aliasing running at 720p natively, it's fair to say that developers have pushed the Wii U to its very limit in graphical capability. Which again is not necessarily a bad thing, as Mario Kart 8 is still a beautiful game. The truth is, Mario Kart 8 (or Super Mario 3D World or Zelda: Wind Waker HD etc.) is a perfect fit for the Wii U. Nintendo most definitely knew this beforehand and opted to make a console that is good enough for Nintendo games with little room to improve for realistic and graphically detailed games. It's why Nintendo games are known for being "cute", colorful and playful visually. Nintendo just doesn't go for hyper realistic graphics in its games, Miyamoto himself said so in an interview with BBC from 2009. (Start listening at around the 26 minute mark:
http://www.bbc.co.uk/programmes/b00p4q2n).
Having all this said, I don't think anyone can truly expect Wii U games to build upon currently released games in graphical quality. It may be the reason why highly awaited games such as the new Zelda game is based around cartoonish graphics we've already seen in Skyward Sword. Keep in mind that there are other ways to make graphics in games visually stunning without significant richness in detail. Take Xenoblade Chronicles (Wii) for example. It's limited to standard definition, but is right there on par with even some PS3 games despite the latter providing HD graphics.
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In the end, I personally don't care about graphics much. Sure, I wish the Wii U was more capable and I'm starting to get tired of the cartoonish games Nintendo makes one after another (Kirby and the Rainbow Curse, Splatoon, the new Zelda). Then again, I'm not expecting a console that is 150-200 euros cheaper to be very capable. The games are fun and the family is having a good time playing together, that's what I expected from my purchase and Nintendo has delivered in my opinion. I may have to buy a PS4 or Xbox One sooner or later, though. E3 this year was excellent, but there's only so many chances I'm giving Nintendo with its cartoonish games. It's time they made something with realistic graphics, maybe a new Metroid or something.