Hello to io9 readers!
This is a strange, neither-fish-nor-fowl sort of issue--it leads directly into FC, but FC also has to be able to function without it (for the inevitable collection), so it's going to end up being a bit redundant, I suspect. Still, the issue number is formally appropriate: the last time there was a JUSTICE LEAGUE OF AMERICA #21, back in 1963, it was the very first "Crisis"!
Cover: This is the first appearance of the "Sightings" banner, which... well, graphically speaking, it sure doesn't look like Chip Kidd designed it. And that would appear to be a Crime Bible in Libra's hand.
Yes, there really was a Mr. Polka-Dot, seen in DETECTIVE COMICS #300 in 1962, and never since. (Fred Hembeck covered that story here, in a piece that also explains the "little girl in the comic strip" joke for anyone who doesn't get it.)
Can anyone identify the provenance of the Einstein quote? I haven't found a source for it that suggests its context.
John Henry Irons is also known as Steel; "boom tubes" are getting-from-here-to-there-instantly technology created by Jack Kirby for his Fourth World comics.
Vixen kissed Superman in JUSTICE LEAGUE OF AMERICA #15, which is also where the "teaching moment" mentioned a few pages later happened.
The race was in JUSTICE LEAGUE OF AMERICA #10; the hole-punching bit was in JUSTICE LEAGUE OF AMERICA #6.
The "secret mission" was infiltrating Cygnus 4019, the prison planet in SALVATION RUN. (How that series dovetails with this issue is unclear--the final issue was supposed to be out two weeks ago, but it's been pushed back to June 18.) The new Atom is Dr. (Ryan) Choi; Ray Palmer, the previous Atom, disappeared after his ex-wife Jean Loring killed their friend Sue Dibny in IDENTITY CRISIS. Jean, who'd been Eclipso for a while (let's not get into it), was last seen in COUNTDOWN TO MYSTERY #4, apparently about to either drown or be eaten by a shark. Ray, as of the end of COUNTDOWN, is one of the new meta-Monitors, but word doesn't seem to have gotten around, so "losing Jean" here probably refers to her being insane and institutionalized in IDENTITY CRISIS.
Ah, here we go. The Human Flame's first and only appearance before DC UNIVERSE 0 was that 5 2/3-page Martian Manhunter backup in DETECTIVE COMICS #274, a story so skimpy we didn't even learn his last name. It was reprinted in that 80-page DC UNIVERSE SPECIAL: JUSTICE LEAGUE OF AMERICA a couple of weeks ago, but if you want a really, really in-depth (and very funny) analysis of it, I recommend the Absorbascon's five-part exegesis, here, here, here, here and here.
Hawkgirl was created by Gardner Fox and Sheldon Moldoff, and first appeared (under that name) in ALL STAR COMICS #5 in 1941. This incarnation of the character first appeared in JSA SECRET FILES #1 in 1999.
Red Arrow was created by Mort Weisinger and George Papp (I think--Wikipedia claims Paul Norris; can anyone set the record straight?), and first appeared in MORE FUN COMICS #73 in 1941. He started using the name Red Arrow in JUSTICE LEAGUE OF AMERICA #7 last year, although a future version of him as Red Arrow had appeared in the sorta-kinda-in-continuity miniseries KINGDOM COME much earlier.
I'm guessing this is supposed to be Twisters (the ceiling lighting fixture is the same), although it'd have been nice if there'd been some common reference for stuff like the exterior, the chairs, the lighting...
I'm gonna need some help to identify the characters here. Panel 2, going clockwise from Libra and the Human Flame, we have [unidentified], Dr. Polaris, the Key, Weather Wizard, Shatterfist, Shrike, Cyborgirl, Ace and... Ten?... from the '80s version of the Royal Flush Gang, and the Icicle. [Thanks to Tom Bondurant and Frank Lee Delano.]
The guy with the arrow on his head is Red Tornado, who was created by Gardner Fox and Dick Dillin, and first appeared in JUSTICE LEAGUE OF AMERICA #64 in 1968. The one with the flaming head, who isn't in a position to even be talking about making that kind of joke, is Firestorm, created by Gerry Conway and Al Milgrom, who first appeared in FIRESTORM #1 in 1979; this version of the character first appeared in the FIRESTORM #1 published in 2004. And I believe this is the first mention of Libra being an "alien warlord"--I'm pretty sure that wasn't suggested in his first appearance.
Clockwise after Libra and the Human Flame: Gorilla Grodd, Ocean Master, Talia al Ghul, Vandal Savage, Lex Luthor. Grodd, created by John Broome and Carmine Infantino, first appeared in 1959 in FLASH #106. Luthor, created by Jerry Siegel and Joe Shuster, first appeared in 1940 in ACTION COMICS #23.
J'onn J'onzz, here apparently traveling via Boom Tube, was created by Joseph Samachson and/or Jack Miller (not the same person; see comments) and Joe Certa. He first appeared in 1955, in DETECTIVE COMICS #225.