I think when all is said and done, Aquaman will turn out to be a good solid movie.
The take on those who saw it that the movie is "good, not great" is not a bad thing. Unfortunately, when people hear "good, not great," somehow in their minds "good" gets interpreted as "average" or "meh," but in actuality good is good and should leave people feeling optimistic.
The reality is that this is probably the most we could have hoped for because the fact is that Aquaman himself is a "good, not great" comics character. He's not the joke that people who only know him from Superfriends make him out to be, but he's never been Superman/Batman/Spider-Man in terms of story quality or importance either.
The other comments I posted above about the movie being "funnier, brighter, more colorful and more adventurous" than Wonder Woman is also good to hear.
If the movie does Ant-Man/Wasp business (over $200 million domestic), I'm sure Warners will look at this as a success and a validation of their new direction.