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.
From monsters' side, Biolabs mobs are surprisingly popular with newcomers and veterans alike. Though Eremes and Flamel stands out from the rest. Eremes' popularity seems to have led to his reappearance in LotS
Comodo's Kafra. Y'know, the green-haired one with Stripperiffic outfit.
Biolabs 2: Egnigem/Laurell, Wickebine/Erende, Erende/Egnigem and Armeyer/Kavach for straight couples. Egnigem/Wickebine and Laurell/Kavach for Ho Yay couples. Erende/Laurell also has a quite significant support due to their matching cards’ artwork.
Biolabs 3: Only Howard/Cecil seems to be most agreed on; the other four are often mix-and-matched. For Ho Yay, Eremes/Howard is the most popular alongside Margaretha/Cecil.
Biolabs 4: Straight pairings are Chen/Celia, Alphoccio/Trentini, and Flamel/Gertie are the default ones. Randel/Celia and Randel/Gertie are often seen just because poor Randel is ignored by majority of fandom. Alternatively, Alphoccio/Gertie and Flamel/Celia for those who like BDSM. Chen/Trentini is just because all official arts released so far always paired them together. Ho Yay pairings are Flamel/Chen, Randel/Flamel and Alphoccio/Flamel. Gertie/Trentini often have a relationship akin of Haruhi and Mikuru.
An unimplemented performance skill 'Ragnarok', for use by a Bard and Dancer in combination, was locked away after initial testing when it proved to not only be a gamebreaker but a terrible means of harassing other players. Once active, it paralysed its casters and for its duration and repeatedly cast random, maximum class/level spells over and over on the surrounding area with no distinction between friend or foe. The skill is still locked away, as the developers still haven't found a way to balance it without rendering it redundant.
For a lot of mid-level characters, Familiars and Drainliars are Exactly What It Says on the Tin for this trope, due to being in a lot of dungeons where they're outclassed by the players and other monsters, taking just long enough to kill to be annoying, doggedly following the player at a fairly hard-to-escape speed, and inflicting blind status with their attacks. Oh, and due to how the evasion mechanics work, they also tend to swarm you right when you're fighting something threatening, thus making your dodge rating plummet to the ground faster than a lead weight. They may have been changed to Passive now, and if they have thank goodness for that.
Worse still, the MVP monster Dracula will summon tons of bats as its minions. When other MVP's only summon a (usually) manageable number of minions, Dracula will summon sixteen. And if you kill all of them (which is VERY easy), he'll just instantly summon another batch.
Loot Drama: Loot and EXP are divided across party members in proportion to the total damage each character has done to the monster in question. Fortunately damage dealers tend to be polite enough to divide the plunder (EXP can be set to be equally distributed all party members are of a similar level). Gets a little more serious after War of Emperium: the prize a guild earns for conquering (and maintaining) a castle is access to their treasure room, which has treasure chests with rare drops that refill daily. Only the guild leader gets to enter.
And to (an admittedly lesser extent), /sit - and a few of the other text commands for emotes.
We can't forget the infamous "WARP TO GLAST HEIM!", either.
The continuous buffs to Suras (Monks' third class), how powerful they are, and nerfs to other classes in Renewal bring us this golden: "Everything is indirect buffs to Sura".
MOSCOVIA CARDS WHEN, or anything similar (originated from the lack of cards of Moscovia monsters, which until this very day aren't implemented yet. Probably one of the greatest case of They Just Didn't Care in this game)
The backstory for Nameless Island. And the explanation on the wiki about the monsters' origins.
The Injustice monster. 'Nuff said.
The Injustice Card◊ turns that creepiness right around into hilarity. Not only while RO is cute is it scary, but while it's scary, it's cute!
The Glast Heim Underprison altogether. You can trace how the prisoners were tortured back in this dungeon's heyday if you look closely at the place. There are guillotines, chairs with shackles, chains on every wall, cages, pools of water, and red-hot pokers. The skeletons are still there, some of them still in the cages or with their ball and chains on. There are also blood splatters on the ground. Now, look at the zombie prisoners, the skeleton prisoners, and of course, the Injustice monster.
Niflheim. That's right, the land of the dead.
Also, Biolabs. This one may be a bit subtle...however, the more time you spend there, the more you begin to think about the...things that supposedly happened there, the creepier it gets. Biochemical laboratory with ghost-like shadowy figures walking around and attacking everything. Naturally It gets worse: All of them are high-(level)class character-avatars (It's even possible to create lookalike characters)with a back-story, all of them being described as good people, especially the priestess who was willing to sacrifice her own life for her team.
It's much worse if you do the quest(s) related to Biolabs. First, you're required to have a pair of handcuffs with you at all times while doing this quest, which causes you to think, what's a monster like that doing with handcuffs anyway. Then, at one part of the quest, you come inside a room with a caged Peco Peco...who can barely talk. Aside from that, you'll eventually learn that people from the slums of Lighthalzen have been slowly disappearing after they supposedly got a job from Rekenber, the owner of the Biolabs. Put them all together and you can suspect that everyone in the Biolabs are the results of an illegal human experimentation.
Speaking of quests, there are others that are also truly horrifying if you pay attention to what the NPCs are saying.
One of the Incarnations of Morroc is a crab-walking, eyeless, bloody woman whose arms seem to be on backwards. Discuss.
Oh and when the Incarnation is killed it sounds like a rope tightening, aka a noose. Pretty unsettling.
The explanation for Rachel Sanctuary, if you do the quest. Hints of people corrupted by a cultlike religion and the pope of the place being completely unaware of what's going on....
That One Boss: Kiehl, at the end of the Kiel Hyre quest. He's an extremely powerful enemy compared to what you've had to deal with during the quest. Granted he IS the boss, but he also has extremely high flee and hit, meaning that he is very difficult, in fact nearly impossible without some very difficult-to-get gear, to dodge with agility classes, and difficult to hit without using a ranged class (all of which generally use Dexterity for their damage, which increases hit rate as well), critical hits, or magic. He's also pretty damn fast and hits like a truck. You had better have a good party to face him, or you will get splattered. He reappears as Kiel-D-01, even stronger than before, though that can be excused as that version is a roaming MVP boss, and as a result never has to be faced for any quest, and can be ignored altogether. At least you get an exceedingly rare and valuable item for defeating Kiehl.
Eremes and Seyren got their race changed from Demi-Human to Demon. Cue crying Assassins who grind in the labs by spamming Grimtooth.
Howard’s element was changed from Earth to Water. Wizards who rely on Storm Gust need to think twice now.
Renewal pushed everyone to insanely high levels (around 120-150 for normals, and 160 for MVPs) with low amounts of EXP per kill. Although 2nd floor’s changes are acceptable, the 3rd floor’s changes received major backlash. Never mind that most people who complained about it were people who were spoiled by other dungeons’ easiness.