*Membership spots not really limited!
. There is a pantheon of seven magical entities, collected of which equate to the powers of Shazam (I assume, anyway -- they all bare the familiar logo and thats the implication). They use their magic to protect the world and "punish those guilty of harming it", which brings us to Pandora.
Remember Pandora and the story at the end of Justice League #6 in which she meets with the Phantom Stranger? Well, it turns out that the two of them along with the first New 52 appearance of The Question comprise what is known as "the Trinity of Sin". Stranger is sentenced for being greedy and is transformed into the familiar character we know, while the Question is apparently an excessively evil person whose memory is wiped and facial features removed. It's all rather grim.
Pandora, on the other hand, is revealed to be the classical version of Pandora, albeit updated to fit the story. She opened some kind of box (which is the skull item you see Batman holding on the cover) that unleashed terrors upon us. She doesn't know what she did wrong, but is considered evil regardless and is cast out.
The rest of the issue reveals what is known as "The Red Room", a government storage facility for the housing of odd objects. There's also "The Black Room", which is the worse version of that and houses all kinds of odd things like a skeleton, what I can assume is the haunted tank and some kind of evil bell, as well as -- you guessed it -- Pandora's box. Pandora acquires and absconds with it, which will assumedly lead into whatever the upcoming "Trinity War" event is that is on the horizon.
Speaking of the "Trinity War", there is a Jim Lee-illustrated future/teaser, which features the gatefold that Bleeding Cool posted, the Justice League fighting each other, a new and evil Green Lantern, Black Adam and Blue Beetle, as well as Element Woman, the Atom and apparently Vibe in the lower left corner. Of all the characters to come back...
Oh ! And also, apparently the war is Batman's fault. I'm not too surprised. Everything is Batman's fault!
The other main thing the issue does is provide continuity. There are references to Justice League Dark, Teen Titans, Batman Inc and Earth Two among other things, establishing the interconnectivity DC has been hoping to have since they started this endeavor.