This was a weak effort by DC Animation (DCA), especially following The Flashpoint Paradox, Doom and Crisis on Two Earths. It was let down critically in several areas, namely in Voice acting, Characterization and Plot.
Voice acting & Characterization:
Superman - Alan Tudyk is miscast in this. He is unable to effectively personify the hero that Superman is and comes off a bit of douchebag in certain moments. The costume he wears in this is not classic...and IMHO it does not look great. They could have updated it without losing the iconic look too much (e.g. Man of Steel).
Supes himself comes off as having a God-complex in this film. He does not seem the noble, self-sacrificing, protect the innocent type of hero we've all come to know and love. He also seems mildly de-powered, though he is easily the most physically supreme of the lot. As usual he disappears half-way through the film and needing Bats to save his ass.
Batman - Jason O'Mara did an OK job with gravitas. Strangely he sounded a bit of a mix between Kevin Conroy and David Hayter (Metal Gear series) - which is a good thing. Bats is well characterized in this film and comes across as the investigative lone-wolf type hero.
WW - Michelle Monaghan admirably captured WW quite well. The character was also kick-ass (with a nice ass!) and very warrior-like. Well done!
Flash - Chris Gorham did Flash well. He was also portrayed well being slightly more mature in attitude without lacking the wisecracks.
Green Lantern - poor characterization. While Justin Kirk did the voice OK, the character lacks maturity and is not a leader-type. This is more the Ryan Reynolds version of Hal at work.
Cyborg - well done Shemar! Super character to introduce in greater detail.
Captain Marvel - welcome back Sean Astin (Samwise Gamgee)! Great job and good character as a boy with Superpowers.
Darkseid - poorly executed. They reduced him to a brawler instead of a master schemer. Insanely powerful though, able to resist the entire Justice League all at once.
Animation style: This is more Korean anime style (as opposed to Japanese anime) which I find more appealing. It tends to moderate between Japanese and Western animation styles. Those who dislike Japanese anime style may find this more tolerable.
Plot: Weak JL genesis plot. Darkseid's invasion play has no clear motive other than to obtain Kal-El. The rest of the heroes are just there because...well...they need to be there to.become.the.Justice.League. Right? ;)
Overall: Poor effort by DCA. I feel they let themselves down by approving an underpar script and miscasting Superman. It doesn't compare favorably to past efforts though still watchable for fans.