These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ear Worm: The main action theme plays for roughly 85% of the game, so it's a damn good thing it's so catchy.
Demonic Spiders: The very first gang that attacks you, on the first screen of the game, attacks faster and harder than almost every other set of mooks in the city. Your best bet for survival is to run immediately into the next screen.
Fridge Brilliance: When you load a savefile, you must also specify a save file for the other main character, even if you decide to start out without him. If you do start alone, the other protagonist will confront and fight you at the end of the tire factory. When beaten, he will drop a usually large amount of money, although in some cases almost no money at all. Now, consider you drop half of your money when you are knocked out.
Game Breaker: Stone Hands, Dragon Feet, or Grand Slam in the NES version. Acro Circus or Torpedo in EX, although your reputation will be trashed if you abuse them.
Acro Circus was a gamebreaker in the NES version also, since the move won't trigger a block animation. Once that got mastered, your biggest fear was accidentally bouncing a box, tire, or trashcan into yourself.
In the NES version, ledges. If a screen contains a brick wall, a fence, or any other ledge you can jump onto, then you can easily wipe the floor there with even the toughest gangs and bosses—simply because the enemies cannot jump. Get onto a ledge and the enemies will typically approach as close as possible, then just stand there, and they won't move again until you touch the ground. So all you have to do is to jump down, punch them once, jump back before they have a chance to react, rinse and repeat. And if the ledge is low enough, you can kick them without even jumping off.
Two Player mode, Grand Slam, each player with a lead pipe or chain. Get on either side of the boss you're fighting, and let the Curb-Stomp Battle commence.
EX: The double chain typically carried by The Eagles, which has insane reach and can be spammed just as fast as any other weapon. It gets even worse with Grand Slam.