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.
Fetish Retardant: Some of the female enemies look kinda hot (Mostly buxom Latinas, clad mostly in fishnets and/or pantyhose, sometimes with styled streaks), but their over-extended clenched mouths when they die... UGH...
ANY type of small enemy in the second game actually, they are frighteningly fast, dodge a lot and attack either by flinging grenades and molotovs or by grabbing, and their grabs inflicts tons of damage. oh, and they ALWAYS attack in groups.
Female enemies in the second game too, they are not as fast as the midgets, but they inflict even more damage and their attacks have outrageous range, meaning you can't easily counterattack, plus they jump and dodge a lot.
Scrappy Mechanic: in Co-Op Multiplayer, if one character moves past the other whilst the other is standing still, he (the first) will do a rolling flip over the other. During this time, the other character cannot move, since he is acting as a fulcrum. This mechanic has caused more deaths during multiplayer than any other element.
That One Boss: The Doctor in Shank 2. He's much much harder than the final boss.
That One Level: "Bombs and Bells". It wouldn't be so bad if not for the fact that Angelo takes an RPG and shoots rockets at you every 5 seconds, making the level a LOT harder than it needed to be.
Last Resort and Deconstruction(7th and 8th levels) of the second game if you play Hard Mode, not much for the enemies but because there is only ONE checkpoint in each, this and the fact both levels are very long and choke-full of enemies and traps with little tequila for replenishing health mean that you'll replay the entire level from the start if you die too soon.