Anti-Climax Boss: The story seems to be building towards a major confrontation between Superman and Brainiac, but Brainiac is ultimately defeated by Lana, whose Mercy Kill of Lex robbed Brainiac of a viable host. Instead of a great battle, Brainiac simply breaks down at Supermanís feet. Superman's true greatest threat is evil!Mxyzsptlk.
Complete Monster: Brainiac. He brainjacks Luthor in the most horrifying way possible, murders most of Superman's remaining friends, and is completely devoid of anything resembling a conscience.
Continuity Lockout: Shades of this, since it references Superman's history, both famed and obscure. However, that's the entire point of the two-parter, to briefly touch on the Superman mythos. Do you really have to have read the Jimmy Olson book in which he became Elastic Lad to enjoy the story?
Evil Is Cool: Despite appearing in a few panels, Moore's revision of Mr. Mxyzsptlk, has remained an iconic rendition of that character. Post-Crisis authors increasingly nerf him because Moore demonstrated quite successfully that the Fifth-Dimensional being is capable of being Superman's worst enemy.
Harsher in Hindsight: The Prankster and Toyman were re-imagined as psychopaths in the Post-Crisis universe.