Nintendo posted the Wii U’s GamePad battery life span figures aren’t very good. The Wii U Gamepad’s battery will only last about 3 to 5 hours, depending on screen brightness. So keep those things tethered, (unless you have two,) or it might just cut your gaming experience in half.
Additionally, there could be framerate concerns with some games while using two Wii U Gamepads. According to Nintendo President Satoru Iwata’s Tweet, “When you use two, the framerate halves to 30 frames per second… At launch, we’ll start by offering games that use only one [Wii U GamePad] and then expand from there.” This shouldn’t be too huge a concern, however–most Xbox 360 and PS3 games top out at 30FPS anyway, so this is a place gamers are very accustomed to, and shouldn’t present an issue in any but the most graphically intensive games.
Don’t let the above get you down, though: one big plus for Wii owners is that you will be able to transfer “everything you bought”, as well as save data, to the Wii U. “How” is another question yet to be answered, but if pressed, I’d guess they’ll tie your Miiverse accounts into your existing Club Nintendo account. Who knows–maybe they’ll even drop Miiverse onto the existing Wii?