So this has been happening ever since I updated my system to 5.1.0. This is kind of difficult to describe...there's a problem with the TV display. It's slightly pixellated, so it doesn't display it's full resolution. This happens no matter what resolution I choose in the System Settings. My TV is a Sony Bravia KDL-32EX500 which I bought in spring 2010. It has no issues of its own, as far as I know. I tried a different system, and the resolution is fine. I tried a different HDMI cable with my Wii U, and the problem is still there. The TV's HUD and UI display in its normal resolution. I doubt it's a problem with the TV itself. But this seems to be a rare issue, if I'm not the only one that has it. Usually, it's not too noticeable, but it's annoying when it is noticeable, and thar I know it's there. It's impossible to see when playing GBA, SNES and NES VC games, obviously, but visible otherwise. At the very most, it is nothing but a minor annoyance and inconvenience, and does not affect my ability to play games. I just wish to play Wii U games in the best visual experience possible.
Anywho, I'll get on with the pictures. I apologize- I understand it's a little hard to tell even with a decent camera, but you should be able to see that the on-screen display doesn't match up with the individual pixels, and it has a weird, blurry pixellation effect.
I really hope this is something that can be fixed, and thank you for your help. I apologize for the size of the images.