In the original series each element was modeled after a different fighting style. But Aang still used Air nation fighting with other fighting styles...because he never really mastered any other techniques...just bending.
You're right of course, but you're talking in a very general terms, and that is hardly applicable to game design.
Even within the show all the definitions are blurred and there is no developed ready-to-use clear combat system that you could just transfer into a game, ATLA isn't Pokemon or D&D. It's a story, first and foremost and an amazing one at that.
This game isn't rooted in real-world kung-fu styles, but it's more in a Platinum tradition with roots from Devil May Cry combat systems (specifically DMC4 with on the fly style-switching). Of course they tried to tailor it into some slight resemblance to the show visually, but mechanically it's Platinum game 100%.
That's why it's hard for people outside of that context to judge what's going in there and see the real depth.
The problem to me is...the bending I saw looked the same...
Let's analyze one scene and i will try to show you the depth, shall we?
http://www.youtube.com/watch?v=EkpG4amknYQ&t=7m02s(if the link doesn't work for you, switch to 7:02 in the video)
We start at long range with water bending blasts from afar:
As the guy on the right start closing in, we switch to Earth which is slower but hard hitting:
and start doing Earth moves stunning the distant guy that we are focused on ("tagging" him, note the the red line with "X" over his head):
Then instantly, mid-combo player switches to Water (actually the switch happened in the last screenshot but the effects of the Earth combo were still there), keeps attacking stunned equalist at the distance and at the same time blast from switching also hits the mook on the right who was just about to attack:
Switch to Air (for some reason):
Two mid-range air blasts:
As the player starts closing in, he switches to Fire (in this game, it's close-mid range style):
With Fire he does combo string with area effect where fireballs are rotating all around us (equalist on the right tries to dodge it):
As the player continues his Fire combo closing in on stunned equalist, he notices the guy on the right who got way to close and switches to Earth to cause "switch explosion" to knock him out:
And another hard hit with Earth (note how the last fireball of our unfinished Fire combo is still in the air):
Our guy is fully stunned and ripe for finishing off:
Not even sure what happens here, maybe when mooks are stunned any attack counts as this kind of crazy finishing move:
We switch to the remaining mook:
Fire attack launches him in the air. Switch to Earth and then Water (because switcher is a circle):
Water attacks keep him helpless in the air, "juggling" him:
Player switches to Air and does Dragon Punch type rising uppercut:
Switch to Fire, two brief attacks, aaaand he's done:
Table with the final result of the fight. The entire encounter took the player 16 seconds:
And it's not even that impressive of a fight to be honest. He only used very basic attacks and combos.