Alternative Character Interpretation: Did Gilda really commit the first few murders or did Harvey's transformation into Two-Face cause her to snap and imagine she did? Likewise, did Alberto really commit all of the Holiday murders or did he just steal the identity after the first few murders and took credit for all of the murders? For that matter, it's hinted that Harvey might have committed some of the murders before he became Two-Face. There's also the question about whether of not Calendar Man really did know anything of the murders or if he was just using it as a grab for attention so he isn't forgotten.
Or, Alberto is a serial confessor (someone with a compulsion to confess to crimes), and that he only attempted the final murder, when he's captured by Gordon and Batman, and that all of the murders are commited by Harvey and Gilda, as suggested by the Calender Man.
Another suggestion is all three suspects committed some of the murders each.
Anti-Climax Boss: Done deliberately with Carmine Falcone. After having been an invincible crime lord for ages, he's casually shot dead by Two-Face. It perfectly highlights how the old school criminals and mobs are powerless against the truly insane and nigh-unstoppable supervillains that are taking over Gotham's underworld.
Narm: Several lines/descriptions are repeated word for word several times such as "Carmine 'the Roman' Falcone, Gotham's untouchable Crime Lord" every single issue. It can get to "Dick Grayson, Age 12" levels of absurdity when read as a collection.
Drunken Riddler. The fact that he kind of looks like a Chuck Jones character in that moment doesn't help.
Nightmare Fuel: Where as usually Two-Face's "scarred" side just looks discolored and deformed, Tim Sale decided Two-Face should actually look like half his face was burned off with acid complete with missing ear and lips. It's not pretty.
Rescued from the Scrappy Heap: Calender Man actually comes off as kind of serious which is impressive given he spends the entire story in a padded cell.
Tear Jerker: Harvey and Gilda Dent start off as Sickeningly Sweethearts. As the book progresses, Dent's obsession with bringing in Falcone strains their marriage more and more. By the end of the book, their marriage is pretty much destroyed and both are dangerously insane.