I am a Justice League Unlimited superfan. I did not expect this would live up to the series (because even the final season of that show didn't), but I was pretty disappointed in this. The story feels fairly empty, but with melodramatic constant narration overlaid on top to give it some weight that it doesn't have.
The story at times is a retread of the incredible A Better World episodes, dealing with alternate dimension versions of our main characters, and the season 1 finale The Savage Time. It kind of picks up from the (amazing) Amazo episode The Return, but it feels repetitive, like it skips Amazo's character growth in later episodes of JLU and brings him right back to the existential questioning, which Dr. Fate and Hawkgirl had helped him with. Yeah this is a really nerdy review, sorry.
The story felt stretched out, and perhaps aside from the Wonder Woman/Darkeid issue, there just was not a lot of imagination or creativity when it came to the alternate versions of the characters. The idea of Batman with a broken back flying around on a little thing was intriguing but he's mostly a tiny drawing in the background.
A lot of it felt like "oh, people like this little thing from JLU, so let's hit it really hard on the head", like Wonder Woman/Batman chemistry, etc, etc. I get it, you want the comic to pick up on some threads, but JLU did this stuff in a subtle way, which is why fans went crazy for it.
The art was great, no complaints there. And the storyline promises something really ambitious and universe(s)-shattering...but yet it felt disposable. I expect something more surprising, funny, emotional and creative from the writers of the actual show.