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.
YMMV: Bloons Tower Defense
Demonic Spiders: Dang, these Yellow and Pink bloons move too fast! Oh, hey, a rainbow bloon... AH! That was close... And why's this brown ceramic thing taking so many hits? Whoa, look at the size of this one. What does M.O.A.B. stand for, again? (*psst* it stands for "hope you've got enough cannons")
In BTD2, it was the black bloons who fit in here- they were the only bloons to be immune to cannons, and since the strongest bloon was the rainbow bloon, cannons were used a lot. The only option against a frozen black bloon were the upgraded boomerangs, which fired in a rather strange pattern. Later installments gave the cannons shrapnel to deal with black bloons.
From BTD5... Regenerators... Regenerators... REGENERATORS... Note that this is a "trait", which means that any bloon that isn't a MOAB-class can have it.
Camo bloons in BTD5. In BTD4, Camo Bloons were a single type of bloon (like blacks and whites). In BTD 5, Camo Bloons are more like a "trait", similar to Regenerators. Every bloon they contain is a camo bloon in itself. Add that to the fact that only a few towers can spot camos, and well, prepare to lose a lot of lives.
And later, you get Camo Ceramic Bloons. Enjoy! Thank goodness you don't face regenerating Camo Ceramic Bloons during main levels.
What's that? Playing on Hard? Well, one of the last five levels involves a rush of BF Bs and MOA Bs...which contain camo regen bloons. Fun!
Game Breaker: Depending on how early you buy them, Super Monkeys and Banana Farms. Also, this game on high levels can be considered a PCU breaker when there's too much happening onscreen. Minimize your window to hear things at their regular speed.
Good Bad Bug: While not exactly a "bug", it is possible to set up a strategy that allows players to repeatedly pop regenerater bloons (of at least black level) for an infinite amount of time. To do this, place a 3-0 Monkey Apprentice & 1-4 Glue Gunners in a location where the wizard can blow the bloons to the start & the gunner can slow them down. The thing is, it'll require a good defense to keep the bloons from overwhelming the player (a Ray of Doom placed so that it covers most of the exit will suffice, as will a Quadruple Spiked Mines set up if the bloons in question are rainbow bloons).
It's been nerfed as of the update that introduced 3 new maps, but it's still possible to gain unlimited money with the use of the Supply Drop ability.
Memetic Badass: The Super Monkey is an elite monkey if there ever was one. He fires faster than a machine gun & can switch to laser blasts when the going gets tough...& then plasma vision when it gets even tougher. He is capable of targeting enemies from far away & can gain even more targeting range. As of the 5th Tower Defense in the Bloons series, he can turn either into a Physical God that can pummel bloons when given the right sacrifices, or a Humongous Mecha that is more than capable of holding its own against massive armies.
The Triangle of Insanity takes the form of Penrose Triangle. It has 3 entrances that double as exits.
There are other levels that could count, but the ones above are probably the most obvious examples.
Really, all the Expert and EXTREME! maps count. Special mention goes to the aforementioned Bloontonium Lab, the oddity that is Clock (the path changes as the clock's hands move), and the downright sadistic Down The Drain (with four short paths).