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.
Continuity Snarl: Because Technos sold the license to pretty much anyone as long as the pay was good enough, most Double Dragon games and media contradict each other and introduce several characters who do not appear in any other media. In general, DD1 and DD2 are canon, and then no one knows.
The Video Games
Anti-Climax Boss: Giga Skullmageddon in Neon suffers a good deal from this on higher difficulty levels: on Normal, he's a respectable challenge and about the right difficulty compared to the previous boss, but on higher difficulty levels he becomes vastly easier to kill since he mainly relies on having a high HP count but low defense: this means that since you basically need maxed out mix tapes to actually make it to him on Double Dragon difficulty, he'll take a lot more damage from your attacks than even basic enemies on earlier levels do and will go down that much more faster as a result.
Audience-Alienating Premise: The concept of Neon being an Affectionate Parody. As soon as the bright colors and retro game lampooning reared their heads in previews, the gnashing of teeth could be heard for miles. Then people played the game...
Game Breaker: In the third NES game, both Lee brothers can stand back-to-back and do the Cyclone Spin Kick simultaneously. If done just right, you get a flashy new animation of their combined Cyclone Kicks. This "Double Cyclone Kick" does five times the damage of a single Cyclone Kick (roughly twice the damage of a throwing knife) and lets you mow down punks and bosses with relative ease. Just...make sure they both stay alive until the end.
The Flying Knee Kick and Hyper Uppercut in the second NES game are both ridiculously overpowered, although the former requires some ridiculously precise timing to pull off properly: you need to hold left or right on the d-pad and then press B and A simultaneously while your character is crouching, which only happens after landing from a jump or while your character is recovering from a fall. Once mastered though, you can mow down mooks and most bosses with relative ease.
The Elbow Attack in the original arcade game thanks to the easily duped AI. Normally the enemies in the game will be hesitant to approach you unless you turn your back on one of them, allowing you to catch them off-guard with the Elbow Attack, which has a decent range and always knocks the bad guys to the ground, regardless of who it is. In the second arcade game, the Elbow Punch's effectiveness is toned down a bit, but its still works to some extent.
Chin in general. He can and will mow down enemies in around three hits.
Good Bad Bugs: The famous trick for skipping Stage 2's boss in the NES version of DD1. Going far enough downstairs will cause Chin to be removed from the level data to save RAM, and the game counts him as being defeated once he disappears.
Mis-blamed: The misspelling of Billy's name as "Bimmy" in the third NES game is not the result of "Blind Idiot" Translation: the game's plot was completely rewritten from its original Famicom release. It's just your average typo.
Moral Event Horizon: When Willy guns down Marian in the beginning of the second arcade game, he goes from being a mere kidnapper to a cold blooded murderer.
The music that plays during the fight against player's own shadow doppelganger in the arcade version of the second game is very eerie and frightening. Not to mention that the doppelganger itself comes out of nowhere and no explanation is given for its existence.
Sequelitis: The arcade versions of the sequels. While Double Dragon II does improved upon the original, in the sense that it was actually an upgraded version of the original, but the increased difficulty, directional-based attack buttons, and recycled stage design turned many players off. Double Dragon 3 on the other hand, replaced the game engine completely (as it was farmed out to another team) and to top it off, the U.S. version made the extra characters, special moves and weapons accessible via item shops that required the player to insert actual credits into the machine.
Seinfeld Is Unfunny: Suffers from this quite a bit, even by beat-em-up standards. Many people who play a Double Dragon game for the first time nowadays will find it to be rather slow and tedious compared to the majority of other brawlers.
Video Game Movies Suck: The movie is a considered a textbook example of this trope, but some of the characters and plot elements from the movie were adapted into the Double Dragon fighting game for the Neo-Geo. And it had Alyssa Milano going for it, so it wasn't completely bad.
On the other hand, the Jean-Claude Van Damme film Double Impact seems to had been inspired by the games and it's an even more faithful adaptation than the official Double Dragon film. Bolo Yeung, the actor who was the inspiration of Abobo's character design (so much that one of Abobo's head swaps is even named "Bolo"), even makes an appearance as a giant henchman who throws oil drums at the heroes.