Erm...I've honestly not seen that big of a difference between 1080p games and 720p games.
I agree that there's not much difference. And when anti-aliasing is used on 720p games, it's almost impossible to notice the difference.
Without the anti-aliasing though, it can seem a tiny bit low-res for this day and age. For example, lately I've been playing a lot of Forza 4 (720p - no in-game anti-aliasing). The game is GREAT... but the jaggies are kind of overly noticeable. Maybe it's just more noticeable due to being a racing game where you are constantly scanning the screen for turns and opponents etc, but yeah, definitely jaggies are noticeably there.
I'm pretty confident however, for the games on Wii U that are native 720p, there will at least be processing power left over for anti-aliasing in most of them.