In Bro-Op, you can revive the other player by standing near their downed body and mashing the button. The graphic they use for this? A pencil going into the spokes of a mixtape and frantically winding it back onto the spools.
Skullmageddon's Bad Boss tendencies, especially when you break his TV's.
"This is coming out of Williams' salary! ALL OF THEM! "THAT'S A PLASMA SCREEN! That's expensive!" "Williams, I'm docking your pay for this!" "WILL YOU STOP destroying my things?!"
While in Skullmageddon's rocket-dojo, Billy and Jimmy end up outside. How do they survive? By holding their breath of course! But not before making a one-liner about there being no air then sucking in air, WHILE THEY'RE IN SPACE.
The title of the Mission 6 boss: The Expensive Experiment Giant Tank. Then you blow it up.
Mission 7's intro:
"What is this? Some kind of lab?" Mission 7: Some Kind of Lab *funky music*
In the countryside, when you destroy the monuments, Billy shouts "Sacrilegious!" in the same excited way he'd shout "Awesome!"
"IDIOTS! Don't let them near the glowing red weak point! (beat) Why does it even HAVE a weak point??? Couldn't you have put a cover on it??!?"
After defeating the Tank, rather than the standard "Billy and Jimmy move forward to Skullmageddon's palace", the tank knocks down the Killacopter, causing it to crash into the tank. This creates a massive explosion that knocks Billy and Jimmy out. Skullmageddon, clearly a little confused, quickly tells his mooks to capture the brothers Lee.
The fourth wall-breaking ending credits song ("Dared to Dream") to Double Dragon Neon, wherein Skullmageddon sings about being defeated by the Lee brothers. The funniest part comes at the 1-minute mark, when he sings "Here's a medal for your victory" and the Achievement Unlocked/Trophy Unlocked dialogue pops up.
He's got plenty of other moments as well:
Right after you press start for the first time in the 360 version, you get an achievement called Skullmageddon's Curse that's worth 1 GS, with the description "Nyah ha ha! Now finish what you have started!": a lot of people like to keep their GS divisible by 5 and the score can only be evened out by getting the achievement described above, which involves defeating him.
Right before you encounter him for the first time, you can find a hair pick from a barrel: if you throw it at him, it'll pin down his hat on his head, making it so that he can't use any attacks that involve removing it until he takes it off while complaining how he could be done in by such a mundane item.
When trying to kill the Lee brothers with a helicopter, he's heard saying "Of course helicopters can fly upside-down; don't be ridiculous. SHRED THEM!" and then proceeds to do just that.
If you pause the game while fighting him, he comments on it in a variety of ways:
"Oh fine, just pause the game!" "I was just about to swing my sword!" "I'm flashing orange so quickly! Quick, just hit me a few more times!" "When I'm about to swing my sword, just punch me and I'll stop! I'll save you a trip to the Internet!"
Skullmageddon's summation of Billy and Jimmy's tirade to rescue Marian.
The "Forbidden Moves" you can perform, which involve one bro crouching behind an enemy and the other pushing them over him.
The Bros have some great lines.
(Knifing an enemy) "Knife to meet you!" (Hitting with a baseball bat) "Hole in one!" (Hitting an enemy with a pistol) "I just hocked a luger!" "Better hold our breath. This is a hard vacuum!" (And they proceed to do just that)
Mecha Biker also has some good lines.
(at the start of the battle) I feel crazy! (after killing the Bros) Hey, buddy, I do what I want! (after having his bike destroyed) Being on-foot sucks!