Contest Winner Cameo: There is a set of stairs near Clayface's cell that lead to a cell where a creepy prisoner is held. He is actually the winner of a GameSpot contest in which the winner got their face and name put in the game. His name, Luke Oliver, shows up on the Joker's party list.
Mid-Development Genre Shift: Arkham Asylum was going to be a Rhythm Game, where the fight scenes would become upbeat 2D dance scenes and you would have to press the right colored buttons on cue, almost like in Rock Band. This whole idea came from these two stories from the developers and from director Sefton Hill. You can still see traces of this in the freeflow combat sections, which do feel rather like a rhythm game at points.note You can mash like hell during fight sequences and do alright, but the game rewards you with stronger attacks and more XP for timing your strikes, not getting hit, and not whiffing on attacks, which requires a more methodical, rhythmic pace of attack.
Name's the Same: Jack White, Joker's alias, has quite the same name as John "Jack" Gillis of The White Stripes, who took the maiden name of "White" when he was once married to Meg White.
For fans of Batman: The Animated Series, including its spin-offs like Batman Beyond and the movies, in addition to Batman, Thomas Wayne, Joker, and Harley Quinn retaining their voice actors, Dr. Whistler is Catwoman, Oracle is Kathy Duquesne, and Dr. Young is Max Gibson.
Tom Kane is Commissioner Gordon and Quincy Sharp, and Mark Hamill is the Joker and Scarface.
Steve Blum voices many incidental voices, so this is expected. It's first noticeable in the introductory sequence with the doctor yelling at a guard to take the Joker away after the Joker offends the doctor by offering to drop his pants.
Wally Wingert voices not only the Riddler, but also the police officer who arrests him once you find all the Riddler trophies.
Most of the dialogue from Harley Quinn's tapes is an almost word for word reproduction of Batman: The Animated Series' Paul Dini's original origin story for Harley Quinn, Mad Love.
Enter the Visitor Center. Check out the Joker statue, turn away, and look back. It's posed differently now!
Maybe that's because it's not really a statue...
The same thing happens to Clayface in his cell; when you rescue the Warden, make a trip down to the cell and he'll look just like Sharp, and if you talk to him he'll claim you saved an imposter. Turn away, and you'll hear him shape-shift; a look back reveals he's turned into Commissioner Gordon.
A clue that Clayface isn't who he claims to be: if you look at him in detective mode, you can't see his skeleton.
Also, after you find Ra's al Ghul, be sure to stop by the morgue again later that night; his body will have disappeared.
The Arkham Care tie-in website lists its partners as Wayne Enterprises for the computer and security systems and Ace Chemical Processing for pharmaeceuticals. The factory was the birthplace of the Joker, when he fell into a vat of acid that turned his hair green, and his skin chalk white. Probably.