As far as where Countdown is heading, what it will mean long term for the DCU, I think they are setting up the New Earth Universe as the 'meeting spot' for all the other universes. It's long been the general opinion that Crisis On Infinite Earths came about when too many alternate universes were mixing together and in the exit interviews from the writers of 52, the universes of the new Megaverse are supposed to be independent of each other.
But New Earth is riddled with apparently extraneous characters from the multiverse, characters who were here from before the Megaverse was created. I think when all is said and done, The Monitors will designate New Earth to take the brunt of any mutiversal instability, with the new Challengers of the Unknown (Jason Todd, Donna Troy, Ion and some other displaced characters, probably) to deal with any problems that arise. From the promos for the new Booster Gold series, Hunter and the Carter Boys will be handling the Megaverse and time, and it kind of makes sense that a team in the present deals with threats rooted in one universe.
As for the events in Justice League and Justice Society, namely the scene from Justice League of America #0, I think that is New Earth's Justice League. In the picture, the Big Tree are looking at the computer display of the two Earths, the picture of the JLA and JSA is in the background. Batman says they have seen alternate universes and a full-on multiverse, and only one Earth has seen a full-on multiverse: New Earth. In the last issue of Infinite Crisis, the JSA make note that they not only remember their New Earth past, but remember a time when Batman and Superman were members of the JSA. Since the Megaverse is new, those universes haven't been in contact each other yet, making New Earth the only possible place for a Justice League with that photo in their base.
As for the "Legion Lost", a friend from the comic store has pointed out to me that Sensor Girl could not exist alongside Karate Kid, at least if they went out on a mission at the same time. Sensor Girl was Princess Projectra after she returned from sabbatical. Why was she on sabbatical? Because her husband, Karate Kid, was killed by Nemesis Kid. What I don't get is, if the Legion already know about the parallel Earths, why do they keep talking as if New Earth is their past? I get that Superman's Legion past was written back in during IC, but Waid's Legion is clearly the Legion of New Earth's future.
Speaking of Meltzer's missteps, this aside is about Amazons Attack! #2. I appreciate Pfeifer's commitment to continuity, but if I have to read one more series with the Justice League standing around calling each other by their first names, I'm going Dark Horse for the rest of my life.
Back to Countdown...Jimmy's encounter with Lightray makes me think the Infinity Man may make a reappearance. He was this super-powerful being that the Forever People could summon with their Mother Box. If Devon is correct, and this Jimmy is displaced, he and the other Challengers may summon him to take on Darkseid, whenever the Kilted Menace makes his play. Or it could be this guy , who was an honorary Legion of Super-Heroes member that gained power of space and time, but lost them during the original Crisis. Maybe he's the "last Legionnaire" Dawnstar and the others are looking for, because Lightning Lad is still in the future, apparently. Then again, Scip has a point about this, too.
I can't wait till the week of Aqua-Jimmy, to tell you the truth. Olsen has been exposed to more powers than Peter Petrelli.