This is why it sucked.
(1) Dialogue Tricks: when you write for cartoon animation for children you write dialogue into the story that you normally wouldn't that basically explains exactly what the character is thinking or doing. Even though it's very obvious and it's not necessary your dumbing it down to make sure that even little children don't miss what's going on. For example in this movie wonder woman keeps saying this rope compels you to tell the truth so now tell me the truth, but she obviously doesn't need to say that because the rope makes them. To an adult it just sounds silly. I mean it's not like they're not going to tell the truth if you don't tell them that the rope is making you tell the truth. Do you understand how silly that is to here? But they decided to dumb down the dialogue and that's my point number one.
2) The villain is not developed. And it's more like he gets introduced and then he shows up at the end. The whole movie was way too much about the team being brought together by Bruce when it really should be like any good really classic really loved movie it should have more about the villain. Point Two, they really failed when it came to portraying a villain to hate in this movie.
3) Point 3 might be arguable but I do believe they fail in the area of how they brought the team together. Instead of the team coming together because they each come to the realize Asian that they should be involved in this fight because it affects them personally somehow they all briefly come together just because Bruce Wayne visits them. In fact I found the way they came together quite boring.
4) Lastly, we never see the urgency of the villain's danger. Who's to say that if our armies got together we couldn't of handled all those flying robots? We never see earth and regular people and our armies try to save the situation in fact we don't see any involvement at all. I mean did anyone even know they were being invaded except for the justice league? They don't really ever say we are now the justice league or we are now all in it together. I don't know but for me I would've like to see the cliché government official saying we're over our heads.
Basically the entire movie is Batman gets everybody together and we fight this villain. And literally that's it. It really is missing a lot that would've made the movie better. The flash and cyborg were nothing more than tokens.
That's my two cents. It's still pretty entertaining. The movie doesn't suck just doesn't stick out it's not fantastic. And for the first live action justice league movie that had so many fantastic actors it should've been a fantastic movie and it could have.