How bad? The gameplay kinda looked button-mashy but the devs said they upped the difficulty so players would have to rely on combos.
Some of the design choices are pretty questionable. Like, if you die, you have to rebuy everything from the item store, but it doesn't save your money, so you can run out of money trying to beat certain fights and then have to go through without items unless you want to redo old levels to have enough money to buy good items and try again. The story is bleh, the cutscenes look bad (almost like stop-motion sometimes) and there's barely any of the characters from the show. Mako and Bolin show up to introduce the game and you never see them again. :\
Gameplay is okay once you figure out the controls (you have like 0.00001 second to counter any attacks so you kinda have to know what's coming beforehand) but it doesn't really feel like you're the Avatar, even after you unlock all your powers, cause there's really no reason to use all of them. You can just smack people with water whips and ice moves and anything that gets close to you crush them with boulders. Air is neat but not really useful and fire is sososo bad. Almost like it's not even there. Pretty much everything about the game is as basic as it gets. Like none of the enemies or bosses are weaker against certain elements, (it's not any easier to beat a firebender using water than it is with anything else, except fire cause dont use fire) so it's just water if you're far away, earth if you're close, air for anti-air stuff and running away.
It really feels like you're playing someone from middle school's video game project. I remember when I was younger they had a summer camp called videogame.net where you'd spend a month and make a little flash game and a website. This is that with a bigger budget and it makes me sad :c