I disagree.

Many of the manga-based anime were produced in parallel with the manga, so they would need those filler episodes to kill time. They were also adapting a fixed media with iconic images, so frozen poses of each character reacting to something "needed" to be included. Same thing with freeze-frame fight sequences, or extended power-up poses. That sort of filler could stretch a single chapter into three or four episodes. And then sometimes they would just make up some crazy shit for a few episodes. Oh no, the heroes have to fight a dinosaur somewhy.

With Avatar, there was no manga, and no need to fill time. The episodes were planned as part of the series, written and storyboarded for television.

Maybe the pacing wasn't always great, and there are some less exciting episodes. Appa was lost for way too long. But that's not the same sort of filler problem that anime struggles with.

It has an amount of filler, sure, but far from your typical anime. As someone that just finished rewatching it while separately watching through Naruto, A:TLA is miles better in terms of less filler.

If you are saying this without ever having finished a full watch through, I would highly recommend giving it another try.

There's a little bit I suppose.

But it does have an end. One Piece has almost 20 times the number of episodes, and is in no danger of reaching any kind of conclusion. It's Coronation Street with pirates.

I can understand why you would say this, but I don’t really agree. There are a couple of complete filler episodes, but for the most part I think every episode is important. Avatar thrives on how good not only its main story is (I think the main story is good, but honestly is weak in comparison to the other things it does so well), but also on the strength of its worldbuilding and character development. Episodes like The Storm, Zuko Alone, and The Beach are amazing for how they teach us more about the characters and show us how they are changing. Avatar Day, The Headband, and The Avatar and the Firelord give us insight into the history of the world, explaining the background of how it came to be how it is, and how the world outside of our main heroes currently function.

It’s a show with just 61 22 minute episodes, telling one unified story. It’s very much not the anime that keeps stretching out plot lines to fill up its seasons, or that stalls for time while waiting for the manga to catch up.