Mostly because it's the Joker running the intercom. Some of his best insults are to the thugs he's sent out to work for him...
"I want everyone to know that I really appreciate all the hard work. You know, watching you guys is like a night in, watching my favorite movie. What was the name of that movie again? Oh, yes. Attack of the stupid bungling idiots who can't find an even bigger idiot running around dressed like a bat! Now get to it!"
"He's taken one of you out! And not in a nice first date kind of way!"
"C'mon guys he's just one man! (Beat) One man dressed like a lunatic and armed to the teeth. *giggles maniacally* Go get him!"
"Whoever kills the Batman wins the grand prize: a night out with Harley! I might even tell her...'eh, why bother? It'll be a nice surprise..."
"*Sarcasm surprise* It's true! You really are the world's greatest detective. How did you do that, Bats? *Normal tone* Oh, what the hell. Get down there, boys! See if he can "detect" being punched in the face."
"What are you waiting for? APRIL FOOL'S DAY? GET DOWN THERE NOW!!
"Paging Dr. Bat! Paging Dr. Bat! Is there a Dr. Bat in the room?"
This exchange between Ivy and Joker over the PA:
Poison Ivy: You will pay, Batman. For hurting my babies.
The Joker: Oh God... Does she ever stop going on about those plants of hers?
Poison Ivy: When I finish with Batman, I'll be coming after you, Joker!
The Joker: Will you really. Well, that's gratitude, isn't it? Women! You give'em presents, experimental chemicals and nice costumes and they still turn on you.
"*dramatic voice* Greetings, Gotham. This is the voice of your new master! *normal voice* ...Oh, hang on, I skipped a bit."
"Joker here! Remember, I am watching all of you, wherever you are. Even you. Yes, you!"
His broadcast after he releases the straitjacket inmates:
Good evening, residents of Joker Asylum! Some of our... crazier guests have crashed the party early. And when I say crazy, I mean real psycho. Word of warning - if anyone sees a dribbling maniac barking at the moon or maybe just purring like a kitten, do your civic duty. Walk up to them, put your arm around them, show them that you care before you wring their necks. *laugh*
"Hey, Bats, go easy on 'em, hmm? For me? ...Oh, hell, what do I care? DO YOUR WORST!!!"
"[regarding Batman's efforts to find Dr. Young's formula] Come on, roll over and die for once. Be your best friend..."
During one of his game over videos, the Joker 'salutes' Batman by putting his hands to his ears and raspberrying at Batman. He continues to do this while walking backwards into the shadows... only to trip and fall on his rear end with both a hilarious look of surprise and accompanying vocals.
Some great lines from the opening sequence, with a dash of Ho Yay:
"Ya know what, I always preferred the good old cavity search. It's much more personal."
"Need to take my temperature? I'd be happy to drop my pants..."
All of the Joker's taunts when you fail are hilarious, showcasing just how good Mark Hamill has gotten with this character.
"I salute my fallen enemy!" *Blows a raspberry* *Falls over laughing*
Perhaps the funniest moment for this troper was watching Joker inject himself with the Titan formula, causing him to transform into a hulking beast. Based on previous encounters with Titan fueled enemies, you're probably expecting Joker's amusing taunts to be replaced with brutish grunts, but instead, after a long menacing stare the Joker begins taunting you in his normal voice.
Obviously not intended, but in the last part of Croc's lair:
Croc: You can't run from me!
Guess what the game prompts you to do.
Also, there are various points where Joker is talking to his henchmen on the intercom. If you take them down while he's talking, he'll realize they didn't answer, get annoyed, and say something along the lines of, "...Goddammit, that's you down there, isn't it Batman."
"I wonder who got to you first, the Croc or the Bat? CROC OL' BOY! You there? No? *annoyed* Pity..."
There's also this when you're in the Sewer treatment plant beating up goons.
The Joker: You want me to send Harley down- *remembers that she's locked up* Do you want me to come down there?
When you rescue a guard and an orderly from the 1st thug in a suicide collar, you get the "Look behind you/I'm not falling for that." bit when you get close enough. The best part is that said thug won't notice you unless you touch him; he's unaware of you otherwise. Feel free to spray some explosive gel and blast the punk into the air.
Batman gets another one after he defeats Harley without barely moving a finger. After she reveals Joker's next step by accident, he just says "Yup, I know". Right after, he lets out a brief chuckle when telling Oracle that Quinn has been "subdued". This Troper was in tears.
This is probably a glitch, but a good one, instead of Batman landing on a henchman and punching him in the face, he might land on his face and punch him in the nuts. Hilarious! It sometimes happens during combat when you do a second takedown RIGHT after a first one.
It's even better if the thug is face-down when you do it, causing Batman to punch him in the right butt cheek so hard, he passes out.
Another possibly unintentional and unscripted but outright hilarious one; every so often during melee battles, the thugs will charge at you screaming and bellowing. If you've hit them enough times and manage to get them then, you can knock them straight out, completely ruining their furious charge.
Likewise, whenever a Titan-infused henchman plows through Joker's other Mooks or even throws the unconscious ones at you.
Also, when you get them to run into a wall and have hit them enough times, you can jump on their backs and control them for a minute, smacking up any goons that get in the way, and even other Titan monsters. Sing it with me: Git along, little Titan...
When fighting two of them, having one throw a body at you but instead hitting the other one in the back of the head with it.
The Riddler accusing you of using the internet after you answer most of the riddles, especially if you were.
The Riddler growing more and more aggravated as Batman gets closer to solving his riddles likely brought a sly smile to many a player's face. Even funnier when Batman solves his final riddle right as the Gotham PD burst into his room.
With the 197th solved riddle, the Riddler calls Batman yet again to accuse him of cheating. On the screen, you see Batman's radio isolate the Riddler's signal after so many calls, find his location, and upload it to the Gotham PD, while he continues to rant. Which only makes his arrest after 200th riddle all the sweeter.
The party the goons throw for you near the end of the game. The sight of all these cold-hard criminals giving you a standing ovation and cheering you on as you walk callously past them is so incongruous as to be hysterically funny. Even funnier, you can beat them up for some XP.
Even better, you can start the fight by casually walking around, spraying explosive gel at their feet. They just stand there clapping, clearly having no idea what's coming. Blowing half the room off its feet in one fell swoop is immensely satisfying.
Or use the line launcher. Where else do you get the opportunity to kick 20 people in the face in one go?
When Batman saves guards from Joker's laughing gas for the first time, there's a lone goon about to fall into his doom. Batman saves him, too, then immediately pummels his head to the ground.
The way he looks up before the boom drops, it's almost like:
Thug: Whoo... Thanks, Bats. Y'know, you're not so bad after- *WHAM*
At one stage, you are unable to reach Harley because of a force field fence. She taunts you, saying that you'll never be able to find Gordon no matter how hard you I'm over here!
When you're playing as the Joker, you can sneak up behind a guy and punt him in the nuts from the back. And then there are the Joker's takedown special moves in the combat maps.... he uses his handbuzzer as one of them, and in another special takedown, two fingers to someone's eyes.
The Joker's infamous line: "Tell me, Bats, what are you really scared of? Failing to save this cesspool of a city? Not finding the Commissioner in time? Me? In athong?". I'm sure that'll haunt Batman's mind for sometime.
Batman: "Harley tried to slow me down. Dropped an elevator on me." Oracle: "Did it work?" Batman: "Of course not."
Also, in the beginning, you go by a TV screen with Warden on it. Joker's response is priceless, especially in the voice he says it in: "Oh look it's my favorite show 'I'm Warden Idiot you'll never escape.' -Joker laughs-" And after Croc says he'll rip apart Batman, and eat his bones, Joker says "That reminds me, I really need to get me some new shoes."
The things that Harley forces Warden Sharp to say when she has him hostage:
"As of ten minutes ago, I have made it illegal to walk on the floor in any part of the asylum. Anyone caught doing so will - this is stupid, I - ARGH! - will have their legs chopped off and perform magic tricks for Emperor Joker!"
When I first heard the Joker planned to take over this nuthouse, like many of us, I thought he was crazy. When he outlined his dictation of a hospital dedicated to ensuring inmates remain loony for as long as possible, I thought he was wacky! Then when he held me down, and razor-cut a new smile onto my face, I decided he had a point. Situated in a small island in Gotham Bay, we stop at nothing to ensure no screwball can fly the coop. Recent successes with Bruce Wayne, a classic case of split personality if I ever saw one, has cemented our reputation as a pioneering slaughterhouse. Our head of research, Dr. Crane, has made no real progress, but seems to enjoy his work, so what the heck. Our new patients' ward is manned by Waylon Jones: I'm assured that he's enjoyed almost every patient sent to him, and boasts empty beds in all his wards.
Batman, when his level of Crazy-Prepared is revealed with a goddamn Batcave in Arkham.
Oracle: How'd you keep this a secret?
Batman (perfectly smug and with a hint of levity he never shows anywhere else in the game): It's me, remember?
Ah, the sounds of Riddler's Villainous Breakdown. Music to my ears. Him demanding that Batman tell him HOW he solved every riddle on the island while Batman remains entirely silent as the police drag him away makes a distinct difference in the coolly arrogant voice over the intercom at the start.
Mark Hamil's over the top laughs. There is one laugh he does I can't put into words, but it's something like this.
Batman: (Deadpan) I'm really not interested in a bunch of flowers, Ivy.
Then, before Harley breaks Ivy out:
Poison Ivy: Harley! You've got to help me!
Harley Quinn: Ivy? Wow, you look like crap! Maybe I can sneak you some shampoo...
Batman is casually, in his own way, walking to where the Joker is in the second to last stage. Then guards began to attack. Cue back handed punches. The best part is Batman is not paying attention to them and still looking at Joker.
Some of the behavior of the enemies in the Invisible Predator sections as they get more and more freaked out at their comrades being picked off one by one. For example, in a battle in the sewer water plant, one guy got freaked out by a loud noise and shot a burst of rifle fire at a generator.
Sometimes, people are talking to the Joker on a video screen, and it's possible to walk up behind them and just stand there for the duration of the conversation. Batman just looks like he's planning to tap the guy on the shoulder just to see his reaction.
And of course, the Joker doesn't say a word. What in any other game this would stick out as an example of lazy programming, here you fully accept that this is the sort of thing the Joker would do (thanks to his many many lines where he expresses his amusement of watching his own men get taken down by Batman).
That mook outside the "party" holding the "Monster Sale" sign. It's the fact that he's completely deadpan while dressed up like a clown that really sells it.
Also, the bouncer mook with the party list who takes a moment to figure out if Batman is on it. "Let's see, uh, 'A', no that's not right... 'B'... 'Bane'... Ah, here it is, 'Batman'."
'Batman' is in fact written in BIG RED LETTERS if you look at it right.
At one point, Oracle informs Batman that the police found one of the bombs that the Joker hid around Gotham. It was full of marzipan and kittens.
Joker's taunts in challenge mode demanding due sportmanship, so graciously voiced: "If you weren't hiding in the shadows, like a little girlll, this would be over so much faster".
After Poison Ivy tells Batman to go to Killer Croc's lair, you have to go back the way you came. A bunch of Joker's men have gathered on the far side of the ravine you had to cross, taunting you about how they'll take you down. You, however, have just received the Line Launcher, which is how you crossed in the first place. It's quite satisfying to fire it, shoot your way across, and hear the mooks yell "What the hell is that?!?" before you plow through them.
It's also quite amusing to pull them into said ravine with the Batclaw
"You know, Bats, I think we should ditch these costumes and run away together … IN YOUR DREAMS!"