"Final Crisis: Legion of 3 Worlds" #1 is dense.
It's the first thing anyone will notice about it. Eight panels a page is pretty much the average and each one is jam packed not just with tons of characters, but loads of them doing things. A major funeral for a major character is given a panel just a bit larger than the UPC code on the cover. That's how much stuff is going on in this comic.
At the same time, this issue (the first of five) is really just a prologue of what's to come. We see Superboy Prime (yes, we can call him that again) arrive at the future, and we see the Legion of Super Heroes we saw in Johns' recent "Action Comics" storyline dealing with the fallout of those events. The other "2 Worlds" Legions are seen only for a panel each. Johns also takes a brief moment to revisit the Mon El crisis that's been threaded through his Superman stories, along with a pretty fantastic moment featuring General Zod and the Phantom Zone.
The series' tenuous connection (so far) to "Final Crisis" is only alluded to in a vague reference to one of the members of the Legion of Super-Villains, Cosmic King, who is said to have "followed an ancient code of sadism and murder inspired by a dark being whose name was never spoken." Three guesses on who that might be (and the first two don't count).
I'm actually pretty impressed at how DC is handling their crossovers so far. As opposed to filling in moments between panels and pages ("Requiem" aside) the publisher instead seems to be treating "Final Crisis" as a kind of ongoing "Sweeps Week" where separate events that have been alluded to for a while (Renee Montoya's The Question finally confronting Crispus Allen's The Spectre, the fallout from the murder of The Flash, the resolution to a year's worth of Geoff Johns' Legion stories) can reach the widest audience under the umbrella of the company's largest Event Comic of the year. It works out well for readers burnt out on the obligations to read almost a hundred comics to get "the whole story" and it also allows more comics to get a piece of what is undoubtedly the snazziest trade dress ever given to a company wide initiative.
George Perez takes a bit of a softer approach for the "Portrait" version of the cover of this issue, but the interior art is another classic example of his usual style. If I had to count, I'd say he drew probably over a hundred individual characters in this issue, from three Legions pictured, to all kinds of crazy future aliens, and even a trip through the Smallville Superman Museum which had exhibits featuring pretty much everyone ever. It's pretty amazingly ridiculous (in the best way, of course).
On the other hand. the story Johns is crafting here looks to be rather grounded. As much as there's tons of cosmic time bending going on, the core theme of the book looks like it's focusing on the idea of maturity. The Legion has always been a team about youthful exuberance and ideology. Johns isn't the first person to turn that on its head and look at what happens when those kids become adults, but he's also framing it in the context of the ongoing story of Superboy-Prime's inability to grow up and face reality. When Superman reveals his grand plan to diffuse the catastrophic consequences of Prime's arrival in the future, Johns makes it clear that there's going to be more to this story than three hundred superheroes punching three hundred villains (although I'm pretty sure there'll be plenty of that, too).