Breather Level: Floor 8, also known as the Mine Shaft. Very few new monsters, none of which are particularly dangerous, it's generally a breeze for its depth and it has a few fun little mechanics unique to the floor. What makes the trope even more appropriate is the fact that it falls right between Floor 7 and Floor 9, both of which clock in as two of the hardest floors in the game.
Boss In Mooks Clothing: Each floor tends to have one particularly strong monster way above the rest, but none is worse than The Great Pumpkinn, who is invisible and thus hard to avoid, likes to roam a lot, summons nasty toxic clouds and hits you for some of the nastiest damage you can find in the first few levels. Speaking of first few levels, he's usually in level 4, give or take a few, so you can bet he'll eat up at least one of your characters.
Several (this being a roguelike, after all), but a special mention goes to the Octos, who occasionally decide to spam painful aethereal bolts at you while you're running away.
Eelys are these early game. They show up in bodies of water and almost always a reasonable distance away from you when you enter their room. They spit powerful acid bolts which can end your run depressingly quick. Usually, they don't show up until level 4, but if you're unlucky, these bastards can show up in one room on Level 1.
Even worse are enemies and traps that can Corrupt your equipment, reducing their bonus to the point of being detrimental to your stats. Special mention should be given to Diggle Archmages, who can Corrupt your equipments at range (others Corrupt at melee range and can be kited around), and come by the dozen in Diggle Hell. Make sure to have 100% Magic Reflection when you face them.
According to the Gaslamp Games forums, the Egyptian Magic skill is this, to the point where it's been nerfed by a patch almost immediately after it came out. It still is incredibly powerful though. Thus, some people reported cleaning level 15 zoos with ease, by way of fully-glyphed sandstorms.
Necronomeconomics is meant to have great effects balanced by debuffs and damage after casting... except the damage can be prevented by just a few points of necromancy resistance, and the last three levels of the skill GRANT necromancy resist and have disproportionately small costs for their effect. The fourth skill on a good tank build allows you to survive in melee combat indefinitely and costs next to nothing, and the last skill does huge damage over time to a huge area that'll take out most groups of monsters in one shot in exchange for a few dozen turns of marginally weaker magic.
Fungal Arts adds chances to spawn mushrooms from dead enemies. Because each chance is rolled separately, its common to get at least one or two mushrooms when the skill is maxed. There is a mushroom for almost every ocassion: strength, damage resistance, mana, invisibility... and you can reroll for free stack of mushrooms you don't want. It also provides two powerful pets that cost nothing but cooldown, and summoning either also creates a large cloud of confusing spores to which both you and your pet are conveniently immune, thanks to the toxic resistance provided by the skill. All in all, it's a powerful skill that helps every build.
Pumpkinns and Witchies can be annoying depending on your setup. Pumpkinns have a high attack and can summon an Area-of-Effect gas over you that, even if it doesn't hurt, it will still drain any buffs that have a hit counter by at least 2. Witchies can blind you if they choose and you don't have high magic resistance, as well as bring back the dead. That being said, neither are too deadly on their own even if you don't have a counter for them.
Any Animal class monster qualifies as this if you choose the Killer Vegan skill. Having this skill makes all Animal monsters peaceful, but they still block your way, are attracted to you, and you cannot attack them or you will get a large debuff to your stats.
Good Bad Bugs: "Features", as they're called, include the generation of items under a shrine square, loss of experience overflow while gaining a level, and spawning multiple zoos on a floor (which are however considered the same zoo, so you'll have to make your way to the other parts to claim your reward).