Unless otherwise specified, descriptions are moving left to right and top to bottom. All characters, unless otherwise labelled, were introduced in Kingdom Come (hereafter referred to as KC).
Page 1. Panel 1. The explosion referred to here is, of course, from KC #1, where Captain Atom was cracked open, killing him and destroying Kansas and much of the Midwest.
Page 2. Iris West, aka Flash IV, aka Kid Flash, is the daughter of Wally West (the KC Flash III and the current DC Flash), and (according to the Revelations supplement) Angela Margolin, who was introduced recently in the pages of Flash. Flash IV's costume is very similar to the costume that Wally West wore as Kid Flash I, while he was with the Teen Titans. The KC Revelations supplement says this about her:
"The young girl in the Kid Flash-derived outfit is the daughter of the adult Kid Flash, Wally West (now simply The Flash). I justify two people running around with the same superhero name by saying that it's simply part of longtime DC history to have lots of guys share a mantle (though the excuse of having alternate versions of Earth is now gone). She would have gained this super-speedster lineage by blood, but her more natural humanity contrasts with her father's evolved, near-godlike state...certain costume details in her lightning-striped pants and booties reflect the design of the original Flash, Jay Garrick."
Page 3. Panel 1. Keystone City, in DC continuity, is the home of the Flash. According to the DCU FAQ, Keystone was in Kansas, where Captain Atom was destroyed, and so it makes sense that Keystone would have been destroyed with the rest of Kansas. This was never made clear in the pages of KC, however.
Page 5. Panel 2. The "Norman" speaking is Norman McCay, the narrator of Kingdom Come.
Page 6. Panel 2. Another change from KC is the costume of Flash III; in KC he was clearly naked except for his hat, while here he continues to wear the costume he wears currently in the DC Universe, along with the metal hat of Jay Garrick, Flash 1.
Page 7. Page 3. Daniel Scheide notes that Barry "catches the meal that Iris is delivering. The meal is exactly the same as the one Barry Allen caught in Showcase #4, his first appearance, after he got his powers...hot dogs, beans, coffee, pie."
Panel 5. Just as Iris West is named after Iris Allen, Wally West's aunt, Barry West is named after Barry Allen, Wally West's uncle and Iris' husband.
Page 10. Panel 4. Barry is referring to the origin of his namesake, Barry Allen, who was struck by lightning and doused by various chemicals, which gave him superspeed.
Page 16. The designs for the tattoos have all the usual superhero insignias, but also that of Spider Jerusalem (from Transmetropolitan).
Thanks to Mark Coale, as always, and to Daniel Scheide.