See I'd disagree that these episodes are filler as they contain important character moments and situations even if they aren't wall to wall plot development. Sokka's Master he not only gets an amazing sword and trains properly to realise his own ability, but the others also realise his worth by the simple fact he isn't around to help them on their journey.

The Blood Bender episode gives Katara a whole new technique that she does use on occasion in later episodes.

Even the Ember Island players episode forces the characters to make realisations about their fate and what's coming in the future.

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To me Filler is separate to just slower "Casual" episodes because it must fit inside a neat little box and not effect the rest of the story. It's most evident in Anime as it means that new characters must disappear by the end and no new techniques or information can be found that would affect the future plot. it literally "Fills" a gap but must contain no substance.