Too much setup, not enough story. The first half of the film has a weird, clunky feel to it, as it seems to jump — arbitrarily — from character to character, trying to keep tabs on all the League before the League is a thing. By the time the heroes finally gather, it’s too little too late.
Also, can I just say that “Mother Box” is a really, really weird name?
Also, also: In much the same way that Wonder Woman was kind of the best thing about BvS, Flash is easily the best thing in Justice League. Having some comedy was nice, but giving a few one liners to Batman was a step too far.