Fountain of Memes: Pick a cover. Any cover. They'll all become iconic (though probably the #1 is Superman holding a dead Supergirl.) Which admittedly the Superman/Supergirl cover wasn't the first instance of Pieta Plagarism in comic covers.
Hilarious in Hindsight: DC's attempt to clean up their messy continuity with a rebooted universe using strict rules on time travel, a winnowing down of characters who duplicated each other, and no alternate Earths. Except they didn't do a clean sweep of the slate (Power Girl and Donna Troy in particular), leaving too many loose threads that required further reboots to try and fix the previous errors.
Jerkass Woobie: The Psycho Pirate is a sicko and a coward yet it is easy to pity him when he goes mad from fear.
"It's not fair. She shouldn't have died for me. Where is he? The Anti-Monitor. I... I want to kill him for this!" It's the horrible punctuation that really sells it.
Wolfman always has embraced the natural cheese that comes with the comic book medium, but here, simply the amount of characters present, and his need to have each one's name spoken aloud by another character, along with multiple characters thinking repeatedly about what their powers enable them to do, ramps up the narm factor exponentially.
That one panel where the Anti-Monitor appears to be air-guitaring.
Uncle Sam's speech to the gathered heroes is really hard to take seriously. Of course, that might have been the point, as right afterwards the line "What a cornball!" is said about him.
When the Challengers of the Unknown realize the darkness in the sky is composed of millions of shadow demons, they're naturally shocked. Unfortunately, June expresses that shock by saying each of her teammate's names all in a row. Seems very unnatural.
Did poor Wally really have to say twelve little no's followed by one big one when he sees Barry Allen's empty costume?
When Peacemaker is hit by an explosion in the final battle, he screams "The pain... The pain!!" It's a line more fitting in a bad drama play.
On the same page, Prince Ra-Man is hit, and his comrade Shade reacts thus: "They've got Prince Ra-Man. This will be harder than I thought, but we will prevail." Doesn't seem like a normal reaction to a comrade dying, except if you don't like him.
Nightmare Fuel: The gruesome death of the Silver Age Flash/Barry Allen, crumbling away like dust as he makes his Heroic Sacrifice. Punctuated with the other heroes arrive and witness Psycho-Pirate frantically tugging on his empty costume deliriously calling for his help.
People being eradicated by the encroaching white wall of anti-matter