No, it's also the originality. Just saying, "It's supposed to be like a myth!" isn't an excuse. Using this, you could (in theory) have the most blatantly derivative film ever and hide it behind myth-status.
Originality is very important. And there are degrees of originality: there's a sliding scale from "literally an exact copy of a previous work" all the way up to "this story is totally original and written in a new language." It's like people are insisting that originality doesn't matter because they can tell it's one of the movie's great weaknesses, and don't want to lend it credence.
Why do we care so much about Don Quixote, or Beowulf? Both are fine stories, but the themes presented in the former represent the first time we'd seen those particular set of themes presented, ever, while Beowulf was the first story we've heard in English. Both are executed competently (particularly the former), but it is their originality that mandates their continued reading centuries or even millinea after they were first authored.
Being first matters. Being original matters. So does execution -- it absolutely matters -- but this isn't an either/or. They can both be important, and both are. Avatar fails on one while succeeding in the other.