Badass Decay: In the first two games, Syrup's modus operandi was to attack Wario, rob him, blow stuff up, or some combination of all three. In Shake It!, it's to flirt with him and then steal his gold when his back is turned.
Possible Fridge Brilliance-induced justification in that attacking him head-on the first two times didn't exactly work in Syrup's favor. Using her brains and womanly wiles over brute force turned out to be a much more viable stratagem.
Breather Boss: Multiple, but Beefne from the first game counts... you just attack him and then throw him off the platform into the instant death lava.
Broken Base: The base is split down the middle on whether making Wario invincible in 2 and 3 was a good move or not. Either it makes the game more creative or too easy. Naturally, this popped up again when 4 undid that change.
A more recent split of the base is whether the Metroidvania approach of 3 or the more linear approach of the other games in the series is better, as mentioned below. Some think the progression and execution of 3 is not as good as the other games in the series, while others think 3 is an overall improvement over 2 and the best game in the series.
Though still considered a good game, Wario Land 4 is more divisive than the titles before it. This is mainly because it makes Wario able to be killed again, there are fewer transformations he can go into (with the available transformations having lost their iconic pieces of music from II and 3), it makes every level a time-trial mission to escape before it explodes, it becomes mandatory to find every single treasure to beat the game, the level designs go back to the more linear ones of the first two games, and it has a much shorter length than the previous titles.
Every game since Wario Land 4 is even more contested. Which one is the 'true' sequel to the previous games? Well, youll get a lot of different answers to that question, as each has different gameplay mechanics and game design. Do you prefer the 3D beat-em-up/platformer hybrid approach of Wario World, the stylus-oriented gameplay of Wario: Master of Disguise, or the more return-to-form 2D platforming of Wario Land: Shake It!? As for whether any of them are even good games... thats pretty debated as well. Wario Land: Shake It! gets flak for being too much like Wario Land 4 except with less new ideas in it, Wario World has a very mixed critical reception in general, and Wario: Master of Disguise is seen by quite a few fans and critics as just plain bad. You can see this by the Metacritic scores of each game:
Despite not appearing in every game in the series, Captain Syrup is very popular in the Mario fandom. Mostly because until the WarioWare series, she was the only female character who wasn't a princess, and one of the few prominent Action Girls.
It was also because she was the first female Big Bad in a Mario universe game. (Yes, she predated Cackletta, the Shadow Queen, the Golden Diva, etc. by several years.)
Fandom Rivalry: With the WarioWare series. As WarioWare got popular, Wario Land became less, and consequently, the latter series became dormant and Wario's appearance in Super Smash Bros. largely focused on the former series. This then sparked an ongoing debate on which series meshed better with Wario and thus should be more prominent. While fans who like both sub-series are not unheard of, the most vocal ones are at odds with each other.
Fanon: The games have never given Captain Syrup's first name, but everyone assumes it's Maple. It does have Word of Dante on its side, though, having been used in a Famitsu article.
Game-Breaker: In the original Wario Land, the Jet Helmet allowed Wario to fly a certain distance by pressing B in mid-air. Wario keeps flying for a fixed distance (at which point it will fly downwards like landing) or until the player presses B again. But, double-tapping the B button caused Wario to reset his flight distance in midair, allowing him to fly as far as pleased.
The Debug mode from the first game (pause the game and press select 16 times) is much worse, as it allow you to manipulate which Wario you have and your number of lives, coins, hearts and even the amount of time you have.
The 10-coin drawn by pressing Up+B was pretty bad, too. Besides throwing it around, any enemy touching it while you held it would be knocked out. This is the same that would happen if you were holding another enemy, only that the coin would not be knocked out of your hands afterwards, so you had an impenetrable shield over your head.
Good Bad Bugs: With some tricky D-pad input, some levels from Wario Land 1 can be skipped on the overworld map, as many SpeedRuns can demonstrate.
Magnificent Bitch: Captain Syrup, the sultry leader of the Black Sugar Gang, is Wario's chief rival and just as proficient as Wario himself. Showing her true cunning in Shake It!, Syrup seemingly puts aside her feud with Wario to help him gain the Bottomless Coin Sack from the notorious Shake King, even putting in her part to help the Merfles the King has kidnapped. After having used Wario to defeat the Shake King, Syrup doubles around to swipe the Sack for herself, revealing she negotiated it as a "finder's fee" from the Merfles behind Wario's back, driving off victorious with one more kiss blown to Wario and remaining syrupy-sweet every second of screentime.
"Seinfeld" Is Unfunny: While Wario Land 3 is still considered by many to be an overall Even Better Sequel and one of the best games released for the Game Boy Color, its Metroidvania style of gameplay (in contrast to the more linear style of other games in the series), the amount of backtracking and several Scrappy Mechanics (particularly the Golf minigame) have caused it to have something of a Hype Backlash in recent years, with quite a few modern players considering the execution, pacing and progression to be a downgrade from Wario Land II.
And finally, the Shake King. His first phase isn't so hard, but his second one gives you a good idea of what Touhou would be like on foot.
To bring that point home, the Shake King's electric ball attack during his second phase combined with some of his other attacks turn the boss battle into a Luck-Based Mission depending on where and how they bounce, and if you die on the second phase, you get sent all the way back to the first phase. Hope you have enough coins for some recovery potions.
Uncanny Valley: Many, many bosses. Indeed, the clown bosses in the series hit the lowest point without even trying, with Rudy looking creepy, Clown-a-round looking really, really artificial and Chortlebot looking somewhat demonic. But then you've got the other bosses, like most in Wario World. The Winter Windster is disturbing. The Mean Emcee is disturbing. Brawl Doll... dear God it's disturbing looking like a plastic cupid doll with overly large eyes, and that's before it turns its head 180 degrees round while laughing. They probably look worse than characters from the Polar Express.