It's structured like a miniseries, so in a way I don't understand the complaints about length. It's designed to be left off and picked up at the checkpoints. Maybe that's cheating since it's one piece, but the result is a story and characters that make way more sense.

My complaints are:

1) that Martian Manhunter is thrown in to no effect. If he's just in a post-credits scene, fine, but why tease him halfway through the movie if he doesn't show up in the final battle?
2) yikes! Leto is terrible. Plus the Knightmare scene completely undercuts the previous Deathstroke scene if you know he works with Batman in the future.
3) speaking of, I like Deathstroke, but he doesn't feel like a threat to Batman after this movie, even without the League involved.