Edifice Complex (Pages 7, 9)
As of May 2018 and Dark Nights: Metal issue #6, once again the Hall of Justice (Page 7) is (according to Super Friends' narrator Ted Knight) "the great hall of the Justice League." Similar buildings housed the League during the 2006-2011 Justice League of America series and the 2011-2012 Justice League International. Alex Toth designed the Hall of Justice for the Super Friends cartoon, which premiered on September 8, 1973.
Maybe it's just us, but Superman's hospital bed (Page 7) seems to share some readouts with Han Solo's carbon-freezing coffin.
With Wonder Woman's D-Clock debut (Page 9), all of the founding Justice Leaguers have now appeared in this miniseries. In fact, all of the "Satellite Era" League has either appeared or been mentioned (the Atom gets name-checked on Page 10) except for the Phantom Stranger. His membership was always a little iffy, though. Of course, William Moulton Marston and H.G. Peter created Wonder Woman, who first appeared in December 1941-January 1942's All-Star Comics issue #8. Gardner Fox and Gil Kane created Ray "Atom" Palmer for September-October 1961's Showcase issue #34.
According to Wikipedia, the headquarters of the United Nations (Page 9) is in a building "designed by a board of architects led by Wallace Harrison, and built by the architectural firm Harrison & Abramovitz." Construction began on September 14, 1948 and was finished on October 9, 1952. As we might expect, it's been the site of many superhero battles on DC-Earth. Among other things, the New Teen Titans fought Gordanian slavers there in their first issue (November 1980), while the future Justice League International saved the General Assembly from a terrorist attack in May 1987's Justice League issue #1. In the early 1980s, Wonder Woman worked at the UN in her secret identity of Diana Prince; and in her role as ambassador from Themyscira she visited there often.
Seems Like Old Times (Page 17)
Jack Kirby created Darkseid, who first appeared in December 1970's Superman's Pal Jimmy Olsen issue #134. Darkseid first fought the Justice League in an October-December 1980 three-parter guest-starring the Justice Society (Justice League of America issues #183-85).
Marv Wolfman and George Pérez created the Anti-Monitor, who first appeared in shadow in May 1985's Crisis On Infinite Earths issue #2, and in full at the end of August 1985's COIE issue #5. John Broome and Gil Kane created Thaal Sinestro, the renegade Green Lantern, for July-August 1961's Green Lantern issue #7. Dan Jurgens created Doomsday, who first appeared in November 1992's Superman: The Man of Steel issue #17. Otto Binder and Curt Swan created Brainiac for July 1958's Action Comics issue #242. Gardner Fox and Mike Sekowsky created Despero for October-November 1960's Justice League of America issue #1.
Elliot S! Maggin and Curt Swan created Superboy-Prime, a/k/a the Superboy of Earth-Prime, for November 1985's DC Comics Presents issue #87. Twenty years later, Geoff Johns and Phil Jimenez turned him evil in time for December 2005's Infinite Crisis issue #1. The Brat of Steel then turned up in 2007's Green Lantern crossover "The Sinestro Corps War."
And yes, Guy Gardner has fought each of these villains and/or their minions. Specifically, he went up against Apokoliptian forces in 1986-87's Legends miniseries and in 1988's Justice League International issues #20-21; faced off against Brainiac in the 1992 Superman crossover "Panic in the Sky"; fought Despero in 1990's JLI issues #39-40; and battled Doomsday in 1992's Justice League America issue #69.