Nintendo has stuck with the "good enough" standard when it comes to console hardware and Wii U won't likely be the exception. Whatever the console lacks in graphics, it will make up for with its unique capabilities. This is similar to how 3DS is not as powerful as the PS Vita, yet it's capable of doing 3D without the glasses. The platform just has to be strong enough to make whatever gimmick Nintendo wants to focus on work effectively and smoothly. Devs will be attractive to it. Of course, the main thing will boil down to whether third-party software sells well that will ultimately determine Wii U's future.
Even if Wii U is essentially like the 360 in terms of its visuals in HD, then at least it will likely have better performance and a part of that probably has a lot to do less with "future-proofing" the console and more to do with making the console capable of handling the seemingly stressful tech involved in getting the console to work well enough with the UPad so things like latency and other tech stuff that's way beyond me, work without a hitch (kind of like how the 3DS has to be strong enough to render the games twice in 3D). For all we know, Wii U may even be capable of handling UE4 if its scaled down. In any case, it can't be as bad as it was with the Wii's graphics where it wasn't only behind the 360 and PS3 but also lacked HD output and even shaders.
I think the reality is Nintendo won't be getting the strongest third-party support. Third-parties have been burned by Nintendo before with its past home consoles to put all their eggs in one nest. Nintendo really just needs to make Wii U compotent enough so that it qualifies for the lowest common denominotor for third-party multiplatform support. Who know,s it may be the lead platform for some games in termsw of current-gen titles, but that's not likely going to be the case once Xbox 3 and PS3 enter the fray. That's naturally where making it easy to port games to becomes even more important. After all, most of us recognize how the third-party exlcusive is becoming a thing of the past. Beyond just making Wii U an easy platform to port games to, Nintendo's already solved the issue of dual analog controls so developing for a specialized controller won't be nearly as much of an issue as it was for the Wii. Even if devs feel like they're ported games need to take advantage of Wii U's controller, most of it seems like it can be implented with ease.
Then there's the online service. More and more it seems to be like graphics are the least thing Nintendo is going to be focused on with the Wii U. Yes, they'll make impressive games and take advantage of their own hardware, but establishing a more flexible online service is a bigger priority. It's going to be easier for gamers and developers to be more accepting of Wii U's lack of visual oomph over another outdated online service. Fortunately, we've already seen signs of Nintendo moving forward with a better online service that's account based and capable of using the controller for voice chat, video chat,and internet browsing using a virtual keyboard on the touchscreen.