At this point in the console's lifecycle, we don't see any games that offer a significant leap over games on the PS3 and 360. Of course, there are a few possible explainations:

1. The lifecycles of the PS3 and 360 have lasted long enough for both platforms to be pushed to their respective limits, while few developers know the extent of the Wii U's capabilities or how they could be reached. In simpler terms, it's far easier to develop a game from the ground up for the PS3 and 360 when compared to the Wii U.
2. Developers are unwilling to put forth more time and resources into a Wii U version of a game just to make it look nicer. Even more so since the Wii U is a completely new console with a nonexistant install base.
3. Nintendo themselves are probably working with outdated graphics engines. For example, Pikmin 3 started development on the Wii, and is likely not entirely rebuilt for Wii U. Nintendo Land and New Super Mario Bros. U are not meant to be graphical showcases, to put it lightly. If Nintendo hasn't shown much effort as of yet, why should anyone else?
4. We haven't seen many major games built specifically for the Wii U. There's ZombiU (which probably wasn't made with a large budget), Rayman Legends (which uses the same technology as Rayman Origins), and The Wonderful 101 (which definitely has some things going for it, but most likely could be replicated on other consoles). We also haven't seen a single in-game screenshot of Bayonetta 2 or Super Smash Bros. 4.
5. The Wii U does not have enough features to warrant the development of games based on anything but current-gen development technology. This is just a possibility, but it's an explaination nonetheless.

Hard to say if we don't even know what the console is capable of yet.

Personally, I think Nintendo Land surprisingly shows some of the best visuals on the platform so far... exclusives wise.

Mmm, I can't quite agree. It looks pretty good, but not next-gen good. I'm more impressed by Rayman and The Wonderful 101, though the latter looks way better in motion than it does in screenshots.

Retro Studios

Maybe. I can imagine Retro's next game being quite a graphical showcase, though they never actually pushed the Wii (or GameCube) to its technical limits. But we can expect a beautiful game, since Retro hires only the best artists in the entire industry.

How can someone be concerned about that? It hasn't even launched yet! i mean c'mon make that question if by 2015 the console's capabilities have been underused, not now xD -- And i agree with bardsong, nintendo land is the best looking game so far, it just feels so full of life.

Nintendo will push the system with Mario and Zelda and with whatever Retro is working on.

if devs are having an easy time just porting the games over to WiiU then there will be lots of room for improvements the coming years.

Pikmin 3 was a Wii game ported over and NSMBU has its stile so it won't show of the WiiU capabilities.

like you said devs aren't willing to spend the extra cash to make their ports look better because they are still in the mindset of WiiU not being for more than the casuals and think their software sails won't be that high on it so why put in the extra effort? the thing is most people who want these game will most likely have bought them on current gen systems so the poor sales will only strengthen the thought of WiiU being for casuals

I hope devs understand this as well as the smaller user-base before jumping on the "WiiU for casuals" bandwagon

ZombiU was one game before it became what it is today so they've had one year to make the game so the time they could have spent on improving the graphics was instead spent on changing the gameplay to fit in with Zombies instead of an alien invasion.

Nintendo does not Retro

I'm concerned because of how underutilized the Wii tech was. Virtually every Wii game looks like they could've ran on the GameCube, despite the Wii being more powerful, and no Wii game is more technically impressive than the GameCube Rogue Squadron games.

I just don't want to see history repeated. It bugs me to see such potential go to waste...

Doesn't really matter considering that Retro is a part of Nintendo, and it's safe to say that Nintendo developers in general employ industry-leading artists.