Robin #10 had Dick Grayson and Tim Drake teaming up, both as Robin. Tim notes that Dick is a far better physical Robin (an acrobat, remember?) while Tim is far more tech-savvy and a hacker.
A villainous one, but Parallax pretty much wins. The universe is dissolved into nothingness, and even though the heroes are in a position to fight him, they don't really have a choice but to let him do his thing. The universe is gone, and he is the only one with the power to actually fix anything. A universe created by a madman is better than no universe at all.
Common Knowledge: It's thought by many people that the "World's End" arc in The Sandman (1989) was a tie-in into Zero Hour, including the DC Wiki and Lewis Lovhaug, expect, as as pointed out by the "Hey, Kids, Comics" wiki, the timeline of events don't add up to that: "World's End" was published in 1993 — a full year before Zero Hour (1994). It's far more likely that the reality storm that caused the events of "World's End" was due to the following arc, "The Kindly Ones", where the titular villains go after Morpheus, attack the Dreaming and ultimately killing Morpheus. This is further cemented by Destiny outright saying that the events would cause a reality storm and "World's End" included an early preview of Morpheus's funeral.
Fanfic Fuel: All those alternate realities the characters visited (not counting ones already previously shown, like The Dark Knight Returns), including one where Alpha Centurion is the hero of Metropolis and one where Barbara Gordon remains as Batgirl after her father was shot by the Joker instead.
The Batgirl of her timeline reveals she and Bruce are romantically involved, and when she attempts to kiss him Batman recoils back in shock. Batman Beyond made them lovers as part of their backstory and Batman: The Killing Jokesees the two end up having a sex scene, both to massive outrage. To make it worse, the Bruce of the mainstream DCU's disgusted reaction mirrored that of many fans to both animated versions.
Batman is sent to a timeline where he was shot by the mugger instead of his parents. Flashpoint (DC Comics) would provide us with a similar reality, only it ended up even worse for Thomas and Martha.
At one point when Superman and the Batmen are guarding a rock concert, the Golden Age Batman expresses disgust towards rock music, long before Atop the Fourth Wall turned Batman hating rock into a meme.
Kyle: Yeah, well... probably bound to happen sooner or later, anyway.
As noted on the main page, this was meant to fix the remaining problems Infinite Earths had brought up, but it ended up introducing more problems which led to Infinite Crisis being used to fix those issues.
Nightmare Fuel: The time-erasing void consumes everything from specific characters to entire universes, and the effects are represented as everything in the comic fading to white, including panel borders, until nothing is left but blank pages. The Action Comics tie-in is especially scary, as Lois Lane fades midsentence and has to watch everything die around her. Other tie-ins are additionally freaky, such as as the Robin and Catwoman ones, where everything fades to white instantly almost without explanation, and in Selina's case, happened so suddenly that she didn't even have time to react.
Older Than They Think: The much-derided idea of Bruce and Barbara as lovers from Batman Beyond and the adaptation of Batman: The Killing Joke? Actually happened here first, as the alternate Batgirl seen throughout the story, introduced in Batman (Vol. 1) #511, was in a romantic relationship with her Bruce (much to, mirroring fan reaction from the animated versions, the shock and disgust of the mainline Bruce).
and Lewis Lovhaug, expect, as as pointed out