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.
Breather Level: Not that the final level, Revolution, is easy at all, but compared to its predecessor, Clinger Winger...
YMMV. Much like its Turbo Tunnel predecessor, Clinger-Winger isn't too difficult once you get a knack for it. While the final third of Rat Race is extremely difficult with even less margin for error and The Revolution is going to be arduous no matter what.
Inverted with 2 player mode in PAL version (in which both players can beat Clinger Winger and continue the game) - in that case Revolution becomes the hardest level.
Demonic Spiders: In the NES version of the first game, pretty much any enemy that can kill you in one hit.
The sharks in Terra Tubes. They chase you constantly, can knock you into spikes or kill you in three bites, and can only be killed after an insane amount of hits.
Good Bad Bug: In Level 10 (the Rat Race), there is a bug that allows you to kill Scuzz before the level is over, and the game will consider the level finished. It's very hard to pull off, but if you can, you can skip the third race.
Growing the Beard: The arcade game amped up the violence while doing away with the nerve-wracking obstacle course levels. It's definitely worth a few of your quarters should you ever stumble across a cabinet.
Hilarious in Hindsight: The Pokémon series now has at least three amphibian pokemon, Poliwrath/Politoed, Seismitoad, and Toxicroak. A small-but-significant number of fans jokingly nickname them after the legendary Battletoads themselves.
LGBT Fanbase: Mostly Big Blag, who appears in gay fanart every now and then.
Porting Disaster: The Amiga version and its CD 32 offshot, though the remixed music of the latter is pretty kickin'.
Scrappy Mechanic: The ability to hurt and even kill your teammate and the fact that if one player Game Overs, they both do are frequently cited as the worst parts of 2 Player mode (other than the whole "Clinger Winger is Unwinnable" thing.)
That One Boss: Since Karnath of the Arcade version can kill you in one-hit, you'd better hope that you're quick enough to dodge his nasty bite or else you'll be out several dollars worth of quarters.
That One Level: Nearly every single level, but several levels particularly stand out:
The most infamous level is the Turbo Tunnel, the third stage of the game, which is when the difficulty goes through the roof. You ride on a hoverbike, with walls and enemies heading towards you. What gets most people is when they place several short walls in succession, as they require consistently timed jumps. Of course, once you finally manage to get through it, some of the later stages turn out to be be even harder.
Level 6, Karnath's Lair, otherwise known as the snake pit. Giant snakes come out of the tunnels in the level, and you need them to climb to the top of each portion of the stage to advance. It becomes stupidly hard, even with a guide, because in the later segments you must drop from one snake to the next, and it must be timed perfectly or you will miss the next snake and land on Spikes of Doom. Also, good luck on the fourth room. Thank gosh this stage can be warped past.
Level 7, Volkmire's Inferno. It's basically a Shmup version of the Turbo Tunnels with your toad straddling a one-man jet. For the most part it's much easier, and it contains checkpoints, too. However, the last leg where the walls come at you faster and faster (again, similar to the Tunnels) can be nerve-racking.
Level 9, Terra Tubes. It's an Absurdly-Spacious Sewer covered in Spikes of Doom, but in this case you're swimming just to make it even worse. It's littered with one-hit kill enemies and fish who will consistently knock you into the spikes. However, what makes most people throw their controller at the screen are the Advancing Wheels of Doom, primarily for being extremely trial-and-error based and the fact that at one point, you have to make it past four in a row until you reach the next checkpoint. Speaking of which, it also suffers from Checkpoint Starvation, which is a problem as it's the longest level in the game.
Level 10, Rat Race. The first two aren't that bad by Battletoads standards, but the third race is an extreme endurance test. You race a rat named Scuzz, whose pattern is running straight until he hits a wall, and then turning around. The problem with the third race is that most of it consists of beams whose openings are at the other end, then falling down to another beam with its opening at the opposite end. Since Scuzz runs faster and falls faster than you, your only bet is to headbutt him on every beam so you can push him up a little and prevent him from falling. The problem is, this requires consistent timing, and you're expected to do this nearly 10 times in a row! There are only two sections of this race where it's a zigzag of falling, giving you a chance to catch up. And even then, the second one is very short. The only way to avoid this is to exploit a Good Bad Bug mentioned above.
Level 11, Clinger Winger. It qualifies for two reasons: First, due to a bug, it is impossible to win when playing two players in the NTSC NES release (it was fixed in the PAL NES release as well as in the Genesis remake), meaning it is impossible to win the game unless you're playing solo (or wait for the other player to lose all their lives so you can continue on by yourself). The other reason is that it's essentially the aforementioned Turbo Tunnel (there is very little room for error), only made even harder, with absolutely no check points to boot. Oh, and you have to fight a boss at the end. Fortunately, they don't make you go all the way back to the beginning if you die to the boss. If it helps any — by pausing the game the moment you come to any turn, you can then make a near-perfect turn every time, making this level a breeze. All bets are off with a second player.
Battletoads — The animated series that never made it past pilot.
Les Yay: Some viewers found a strong homoerotic subtext the scene where the Dark Queen grabs Princess Angelica, who tries to tries to push her away even though the Dark Queen is grabbing her closer.