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.
The orb market, either it's a great replacement over using gold, or it's too complicated and grindy.
The developer's ah, controversial decision to stick with non-instanced loot.
Now that they've implemented an option for permanently allocated loot, it's more broken between "Stop Having Fun" Guys who think that free-for-all loot should be the only way to play and that the developers shouldn't have added the option, and everyone else who just plays with the option they prefer.
The skill-tree has always been a split between those that love it for players being able to control how to evolve their character, and those who think that the skill-tree is too simple and lopsided towards certain builds to bring any depth or creativity to the game. A prime example being when a player works around the skill-tree to pile up on the skill points that increase health over experimenting with any of the other skills, because piling up on health is considered to be one of the more powerful builds in the end-game.
Demonic Spiders: Rogue Exiles are hostile NP Cs that are exiles like you and have a chance to spawn pretty much anywhere. They have access to the same skills as you do and can be very dangerous, especially in the higher difficulty levels. Bumping into one unprepared can lead to a swift death.
There are a few monster mods that can down players quickly. For example, Corrupted Blood inflicts the bleed status effect and can sap your health in seconds unless you have a flask with a remove bleed effect on hand. Monsters with Reflect Elemental Damage can also be a huge pain for casters relying on elemental spells. Piling on a load of Storm Calls or fully charging a Flame Blast can lead to an Oh Crap moment when you discover that the monster you're about to nuke is going to reflect that right back at you.
Disappointing Last Level: The Sceptre of God. Six floors of the exact same types of enemies with no variation aside from a miniboss on the second and sixth floors, and unlike the Lunaris Temple, the environment remains exactly the same no matter how far you are. Oh, and the Waypoint is located on the third floor, so if you die to Dominus and forgot to use a portal scroll, or if your game disconnects or crashes, then you have to slog through three levels of the same damn enemies just to get back to the final battle.
Luckily, the developers implemented a checkpoint system that allows you to revive just before the final boss fight. Still doesn't help if you crash, though.
Dueling Games: With Diablo 3 (and perhaps Torchlight 2) as the prime action-rpg of this time-period. Marvel Heroes is sometimes brought into the conversation as well.
Early Bird Boss: Most sub-bosses in Act 1 on normal difficulty, primarily Hailrake (see below).
Holy Shit Quotient: The typical reaction when someone sees this game's skill tree for the first time. Fans call it a skill forest for a reason!
Moral Event Horizon: Given what you see of the results of Piety's... "experiments" inside the Lunaris Temple, she'd crossed this point long ago.
Nightmare Fuel: The game gives you brief flashes of horror over the course of the first two Acts, before turning it up to max and leaving it there in Act 3. Special mention goes to the Lunaris Temple, especially Level 3. The monsters there are the results of Piety's "experiments", the entire area is flooded with blood, which will spawn blood elements to try and kill you! On top of that, the area is full of increasingly high amounts of bodies in various conditions, and the ambiance and music is very unsettling. Did we forget to mention the fact that there are live people floundering in the blood in some areas?
Oak in Act 2's bandit quest. He's incredibly strong, and he has a leap attack that instantly sends him to his target. He's even harder to deal with if you like using a lot of fire skills. Expect to find yourself joining random parties just to deal with him.
Merveil, the end boss of Act 1, is this for some players, especially those who are new to the game. At first, it seems like she just attacks the player with many powerful frost-based magic attacks, but upon defeating her, her true form appears and she not only completely regenerates her health, but also becomes a summoner-type boss (alongside being an ice-mage) that will endlessly spawn Mooks to constantly annoy you; some even being suicidal-bomber types which will instantly destroy a chunk of health if the player let even one get too close. To top it all off, she has a shield that will regenerate if players hold off on attacking for too long, and in the second fight against a summoner-ice-mage, expect it to happen A LOT. Long story short, players are in for a long, drawn-out, fight against Merveil if they're not stocked up on cold-resist, or they're short on any area-of-effect type attacks to keep the summoned pets at bay.
Hailrake is brutal in both normal and merciless difficulties. In normal you fight him at about level 2 when you have no real gear, skills, or passive points and you're just getting the hang of the game. His primary ability is both extremely powerful and fairly counter-intuitive if you don't already know what it does (at short range it's harder to dodge, but at long range it's 7 times more likely to completely freeze you which is more or less a guaranteed kill at this level). On merciless you know what to expect, but the map he's on is level 58 and directly adjacent to level 53 maps with no warning that's the case. He is in fact higher level than the mid act boss Brutus and about the same as the end act boss Merveil.
Players disappointed with Piety's relative ease have had complaints answered with High Templar Dominus. Boss Rush as first and second phases, Flunky Boss while being no slouch himself in third and fourth, and in said fourth phase, he can call a bloody death mist that instantly kills you unless you stand right next to him and his powerful melee attacks.
Your first fight with Piety in the city ruins. She spams chain lightning and this one spell that causes a huge ball of energy to careen across the screen firing thunderbolts at anything nearby. If you have low lightning resistance, she's a nightmare.
The Bonus Dungeon carrion-beetle boss, the Sunburst Queen. Powerful fire-spells coupled with the the Queen's never-ending spawning of 50 to 100 suicidal-bombing carrion beetles makes for a rather difficult fight. It's almost like dealing with a fire-version of Merveil...only a hundred times worse!
That One Level: The Battlefront of Act 3; filled with Black Guards galore. The worst part of this zone is when you come across the mage variants of the Black Guards who constantly use a thorn-type of spell that causes any form of damage taken from the player to be copied back at said player. It's absolutely brutal to player builds not dumping their skill points into increasing their health or shield.
The Marauder gets some flak as this. As the pure Strength class, he focuses on powerful melee attacks and armor that is purely defensive. The problem is that, towards the end of the game and especially the higher difficulties, you pretty much need some form of ranged attack and evasion/an energy shield to stay alive without having to guzzle potions every other second.
Several spell gems usually get this reaction due to higher level gems being perfect upgrades in comparison. A good example is the Fireball gem you acquire right at the start of the game, but then gets replaced by the infinitely better Firestorm gem that can be used a couple level-ups later.