With an outstanding art design and a smart optimization of the console possibilities.

I don't think it could compete with PS4 visuals. Even if they moved it to NX, it probably wouldn't. Though usually, visually stunning graphics is not what they go for, but gameplay. Their games still look great though, but nothing comparable to PS4/Xbox visuals.

I'm not sure what this means. That Zelda on the Wii U will look as visually detailed as the most cutting edge game on the PS4? It won't. That's fine, though, as it's probably not supposed to look that realistic. It's a fantasy world and I doubt they'd build to that level even if they could. The gameplay video with the horse was nothing specially visually but it's the style that's important with a franchise like Zelda.

Cell shading.

Um, it won't look better than PS4.

Its not competing with  visual of realistic artstyle but of uniqueness and a different type of artstyle like Skyward sword and windwaker. It will look great like mario kart 8 looks great

It will follow an artstyle that best suits the hardware. If MK and Smash U looked great on the WiiU, Zelda U will too.

Purely visual?It wont be able to compete.But a game, to be visually appealing, dosnt rely purely on visual, but also on the art direction.And with an excellent art direction, Zelda U could easily "look" better than most PS4 games.Its just a matter on the artstyle and how to use it to its fullest

I doubt many expect it to match the PS4 graphically (and frankly the minority that do are not the type of people you'd ever want to discuss graphics with :p), but outside of specific visual features there's nothing stopping it excelling in the art department.

I'm confused. Who said that it would? The way the question in the OP is phrased, it seems like you are challenging someone who said it would look like a PS4 game. I've not once heard anyone make that claim.