Well, I never saw Eragon. But here's what a guy who heard The Last Airbender sucked thought about it when he saw it:
Haha, oh wow. This movie is genuinely awful. You can even strip out the elements that could have just been awful because of the initial bad idea of turning a season's worth of TV episodes into a single movie -- you'd still see that everything left is awful on their own merits.
The biggest slap-your-forehead-this-is-dumb problem is that the element-bending dances are often accompanied by no actual element-bending. You're just watching a shitload of Tai Chi, watching your patience bending. Even when this IS accompanied by special effects, the bending lags FAR behind the actions of the bender most of the time (To the point where they will complete their bending dances before any CG shows up). Despite how nice the post-production effects look, they just don't decently affect the real world around them. Sometimes they have none, which is pretty shitty for air-bending (Because, well, real air blows hair around). It's a problem.
But the larger problem is that the entire movie is a poorly-edited mishmash of exposition voice-overs, exposition scenes, emotionless scenes to show off scenery, scenes of Prince Zuko being angry (The only mildly-professional emotional displays within the film), and scenes of characters we know literally nothing about pretending that they have forged bonds or grown as individuals sometime during the five weeks that they tell us we just missed. The editor fucking sucked, and does things like cut a voice-over of Katara talking about Aang (By name) directly in front of a scene that starts with her asking Aang what his name is (Which is... distressingly far into the movie, yet gives him only his second or third line).
It's really amazing! Everything I thought that the director MIGHT have trouble with -- you know, this being such a huge leap from the type of film he knows how to do -- and everything I thought seemed bad about the casting, turned out to be even worse than anticipated. M. Night seems like the kind of guy who only juggles two balls at a time with any success, and only when those two balls belong to extremely talented, successful actors. This movie had too many balls, and the balls were too challengingly hairy for that amateur. I really feel that I must stress that there is no ball left un-dropped in this film. It is as stilted and laughable of an affair as you could ever wish to avoid.
Ethnicity choices really felt weird. Not just in looks, but the strange influences on pronunciation as well (I assume that was meant to be some sort of "ethnic" choice, at least). "Ovatar"? A single black guy in the entire movie? It's absurd, although speaking as a fan of the show (Since these could be construed as fan-related complaints) not as bad as how none of the characters except Prince Zuko and Aang (Barely) are even remotely true to their original counterparts. This is mostly because none of them ever have scenes where they're allowed to be personable or have a good time.
Too bad.