1080p output doesn't mean anything
Both the 360 and PS3 have 1080p output too but usually use 720p (or less) because most games don't afford them the CPU / GPU budget or memory to let them go any higher. So the claim 1080p is utterly meaningless. What matters are the hardware specs, particular the CPU, GPU, memory and the bus connecting them all together. When those are know people can make a fair estimation of its likely performance.
It if turned out the Wii U had a 4ghz quad core CPU and a NVidia 9 series derived GPU one would expect the system to scream. If it turned out to be a bollocky 1 core hyperthreaded CPU with some off the shelf embedded GPU one could expect it to stink on ice.
The devil is in the details but Nintendo have hardly been known to push the boat out in the past. Usually they ship extremely lacklustre hardware and coast on a gimmick and the indication is they intend to do the same this time. It looks like the controller is half a DS XL screen attached to some controls with something equivalent to the PS3's remote play to enable the screen to mirror or augment the TV's main screen.
I expect the console will have "just enough" processing ooomph to exist on the same tier as the 360 & PS3 and share title ports and that will be that. Which would be fine if the thing came out 5 years ago but it isn't. It will be appearing at about the same time that the next generation of consoles appear.