To be clear I did not have high expectations for Justice League. I was thinking it was going to be Batman V Superman but with more characters and plot turns. So worth watching for the action, but little else.
But boy was I wrong, it was very good, surprisingly so.
Going through the list of things I though would tank it but it did well instead:
First they had to introduce three new characters, Aquaman, the Flash, and Cyborg. I though this would drag things down or the characters would end up flat because they could not afford enough time to flesh them out.
Neither happened, little time was spent on them but it was time well spent. They showed the characters, they did not just tell us about them. We did not get much backstory but we did get to know them as characters. The actors of all three characters deserved all the money they got and then some.
Second I thought it would have a consistent joyless tone through the whole movie. While the start was kind of joyless, it was not near as bad as Batman V Superman. And unlike in Batman V Superman the tone at the start helps the movie. Its a movie about making things better, so starting joyless works (like horror movies starting happy). Things get better as the movie progresses and there are even jokes (good ones even).
The third was that the plot would be too complex and would twist too many times (looking at you spiderman 3), and they would leave you in the dust trying to deal with that AND all the new characters.
That did not happen partly because they knew their limits and kept the plot simple but workable. The bad guy was not complex, as he is not the point of the movie, he starts the ball rolling but he is not the star.
While simple the stakes are shown to be very high, and the bad guys power is never in doubt. While I do hope for a more complex one in the next movie, in this one the focus had to be on the league.
One final note is I really like where they are taking Batman. He is old and he shows it, they did not have him take on things that a normal guys should not be able to. The movie as a whole can be seen as him trying to hope, trying to be more like superman. He fails sometimes but he tries, again and again. THAT is a batman I want to see on the big screen.