well, not really spoilers, since you guys probably have read the comics already (i haven't), but i have watched the direct to dvd movie already (bootleg, some parts were in black and white, but briefly only, because it's like a promo copy).
It's like a more violent justice league episode.
Some scenes show blood splattering on a character's face (and lots of it), like when Hal had to shoot a Korean point blank to his head.
Superman was not really a factor (i do not know if that is how it was presented in comics), he went down first against "the Center", but he did give a pretty good speech to unite the heroes and the military.
Batman gae the best line (staring down Jon Jonz-human form)--"i do not know who or what you are, but my instincts tell me you can be trusted. But just so you know, i have a $70,000 sliver of radioactive rock to take down the guy from metropolis, with you all i need is a penny for a box of matches". BADASS!!!
The Flash has the most number of kewl scenes, like taking down Captain Cold (the only villain in the story, "grodd" was a robot); exposing "The Center" to Ray Palmer's shrinking ray; running with sonic booms.
Hal only used his powers once, containing the explosion of "the Center".
Wonder Woman and martian manhunter were not much of a factor, at all.
Batman made a costume change right before the inal fight (when he took in robin).
King Faraday died. He grabbed two grenaides and exploded em inside the mouth of a dinosaur.
Funny scene, Martian Manhunter copied appearance of BUGS BUNNY.
Cameo Appearances by:
End scene: US President talking, shows pictures of heroes and villains like the one in the trailer (actually, exactly like the one in the trailer).
I will say, it's very good. The voice actors selected are perfect. A marked improvement over the Superman/Doomsday dvd movie voice talents.