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.
Awesome Music: In a nod to Diablo II, when you defeat the Overmage, some of the music from the first game plays in the background.
Big Lipped Alligator Moment: The side quest "The Morden Riders" may come across as this. While the other side quests have some sort of connection to the overall storyline, this one does not. Especially since after it's finished, the characters never mention it again. Or indeed, at all.
Demonic Spiders: There's an enemy if the final dungeon of DS2 that casts a curse on the whole party which deals a lot of damage over time and can easily cause a Total Party Kill. This basically forces you to bring a nature mage with the curse immunity buff.
Disappointing Last Level: The final level of DS II, Zaramoth's Horns. The good news is that there's a lot of good loot and experience to be had. The bad news is all that grinding can get rather boring after a while.
The final dungeon of Broken World has you traverse a seemingly endless series of identical rooms until you reach the final boss.
Drinking Game: JEYNE KASSYNDER is mentioned so often, enthusiastic players turned her into a drinking game.
Narm: The beginning of the first game can count as this. Think of it: You are a peasant fresh off the field, dressed in nothing but rags and armed with nothing but a tiny knife, and you are able to slaughter entire packs of burly Krug (who have already sacked your entire home village) with barely an effort.
Some of the dialog in Dungeon Siege II could get pretty goofy, such as this one Taar says in the second half of Act I: "You must take care when talking to strangers. You never know when someone is lying." It would seem that Taar took the expression Team Mom a bit literally.
Nightmare Fuel: The Bound creatures that make up a majority of the enemies in Broken World, especially since they can remind one of a notorious episode of Fullmetal Alchemist. Plus there's the Dark Wizards and the Familiars, and to some people, pretty much Broken World as a whole.
Of course, that's not to say that the prior games didn't have scary stuff of their own, but fortunately, in their case, it was safely distributed here and there. Broken World, on the other hand, just piled it all on.
There is also The Pit of Despair from the Utrean Peninsula. No enemies, near total darkness, and it can take hours to complete, since you are constantly backtracking.
Porting Disaster: Despite being a simultaneous release, the PC version of DS3 has serious interface problems, specifically requiring auto-lockon instead of the mystical, sophisticated computer-only peripheral known as a mouse in combat.
There has since been a patch that added keybinding and the capacity to use a WASD layout. However, the targeting system has not been patched.
That One Level: The goblin workshop in the first game will likely be the first stretch that forces you to backtrack to the nearest vendor for more potions. While you've seen a few boss or midboss monsters that do massive area-effect damage before, the workshop is the first place with entire packs of them, often ambushes you with huge packs of them at the end of elevator/conveyor rides where you have no reasonable escape route (or from behind), and forces you to take them head-on in narrow corridors where you're unable to turn their blasts to the side. Heaven forbid you're playing on Hard — healers are mechanically incapable of keeping up with the damage flow of more than a couple of them, summoned monsters with three times the HP of party members dissolve in seconds, and you'll often have to clear rooms via ten minutes or more of tedious hit-and-run or summoning creatures around corners.