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.
Anticlimax Boss: Sometimes the Alchemist's imps/golem/pet will go charging into the Boss Room before the pre-boss cutscene is triggered, causing the boss to explode at the beginning of the fight from all the damage dealt before the cutscene even started. Kind of makes them skippable bosses.
Demonic Spiders: Each class has its own Demonic Spider in the last area. For melee specialists, the Dragonkin are major threats, as their fire-breath is short-ranged, constant and very damaging. They often come in groups and can take a lot of punishment. For ranged specialists, we have Dark Zealots, who throw down enough damage to one-shot anyone who is even slightly squishy, and they can do it in two different elemental flavors, just in case you're thinking of bulking up your resistance to their attacks. These attacks scale in power with your level, which means that a lucky shot can kill a Squishy Wizard even after they have leveled up enough to shrug off anything else. Oh, and they are also Mook Makers. They can summon a Zerg Rush of "Enslaved" skeletons who out-DPS you, even if you're tanking.
The Vanquisher "Trap" skills are basically sentry guns, and they fire really fast. While the damage on bosses is negligible, you can spawn enough of them to whittle him to death, and they'll eliminate the Mooks he comes with too. Plus they make Kiting viable, which is almost the only way to survive in the Black Palace and bonus areas based thereof.
The Vanquisher's Explosive Shot has good damage, a good firing rate, a huge blast radius and knockback. It also travels farther than the player's view radius and blind spraying into a suspicious unrevealed area can snuff out entire groups of enemies plus a boss or two before they appear on-screen (which prevents them from reacting). The damage scaling isn't too spectacular at higher levels but at the same levels mana consumption becomes less of an issue.
The Alchemist in general is fairly broken. So much that some of his skills, like the Ember Lance, had to be nerfed, and he's still very, very powerful. The only balancing factor for him is that he's squishy, and even though you get potions by the tons, they go down lightning fast when things start getting hectic.
Good Bad Bugs: if you were equipped with a ranged weapon and encountered a Chest Monster, instead of moving to the chest, your character would simply open fire. For good or ill, the sequel fixed this.
Most Annoying Sound: "Your health is low!" "Your mana is low!" "Your pet is injured!" "Your pet is fleeing!" If you're spamming the 'group health' spell you can expect to hear the sequence of "Your pet is injured! Your pet is fleeing!" repeated over and over again.
On that note: "My inventory is full"/"I am overburdened"/"I can't carry any more".
The sound of your Golems marching. It will drive you mad.