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.
YMMV: DC Universe Online
Crazy Awesome: T.O. Morrow, if not solely, for being able to screw over The Joker of all people and get away with it.
Demonic Spiders: The Paradox reapers. They could appear in virtually anywhere in the open world, and could kill new players before you could blink.
The Joker androids. They have a powerful explosion attack that reduces their health and explode when they die. Even if they died due to the explosion. Both, combined, can take away 90% or more of a hero's HP.
The Green Lantern/Sinestro Corps members in Metropolis. They swarm, often have shields you have to break before doing normal damage, and can send you flying backwards (or pull you towards them, so you can't easily escape) with ease. The Green Lantern centric duo is even worse in some parts.
The craziest examples have to be the creatures that comes with the tier 4 (highest level) missions. These can kill moderately experienced max level players out in two to three hits.
Fridge Brilliance: Why are the iconic heroes standing around tutoring young heroes instead of out there fighting the surge in crime? Their rogues gallery just multiplied. They don't have the time to fight crime and still have a normal life. And they do get out of the office occasionally, particularly during the "mentor arcs" where you join them to fight their opposite number.
All the pre-order copies have different bonus items depending on where you bought them. Three guesses what the item for Amazon.com pre-orders is, and the first two don't count.
A lot of the "briefings" and whatnot you can discover throughout the game have snippets from something called "Daily Planet Live", with Lois Lane giving a news report. Why would the Daily Planet be giving a live show? Well, the New York Times and Washington Post got their start as newspapers but have had to diversify in the last decade in order to stay in business with newspaper revenue going ever downward! Of course the Daily Planet would have to branch into other forms of media, too, if it's going to survive in the Internet era!
Plus, the Daily Planet is currently Bottled, so it's not like they could really get access to their presses at this point in time.
Why did the Arkham villains decide to capture The Joker? Remember how you betrayed Mr. Freeze and stole the diamonds for The Joker while leaving him to be sent to Arkham? Remember his alluding to him having his revenge?
Most Annoying Sound: Fighting Demons in Metropolis. "How Daaare You!" "The Wrath of a hero! Delicious!" "You Will Regret That!"
Harley Quinn is easily the worst offender. Her boss fight consists of her making the same hammer puns (about 5 in total) every time she attacks, which is every few seconds.
She's just as bad on the villains' side as a Distressed Damsel, just replace hammer with bird puns. Robin futilely tells her to shut up.
The Jokandroids, who have only two death lines that they utter every time
Not to mention the HIVE drones. "Sszzszzzwarm them!" "That STINGS!" and other (though not many) Bee-related puns repeated again and again.
Bruno Mannheim. Not only are you completely outnumbered, but Bruno has a beam attack that grounds you, is impossible to dodge, and shaves off so much health.
Black Adam. Aside from his Word of Power being enough to shave off half of your health, absorbing Zatanna's and (to a greater degree) Giganta's powers is considered him being at his most dangerous. * Precisely Zatanna!Adam's lightning attacks and Giganta!Adam's leap attack can reduce your health very quickly.
The mission to rescue Supergirl from the metahuman hospital has its first encounter with Doctor Psycho early on, but it is a 'game of hide and seek' where you have to roam around the room until you are close enough to see him only to be able to hit him a few times before he vanishes again. There's no threat to you, but it takes time and you really don't do any damage to him either. And it's one of the challenge modes at 30, later on. It's one of the most difficult ones due to the encounter after that little game of hide-and-seek.
Batcave: Inner Sanctum. If you queue up to this raid, you'll get a group that is stuck at the Armored Robotic Custodian about 90% of the time.
It's so insanely difficult that SOE eventually have to nerf it.
Fortress of Solitude 2: Power Core. There's a portion of the mission where your team will have to split into two groups to proceed which includes taking on two Brainwashed and Crazy Kryptonians (one for each group). Good news: You only have to take off around half of their health. Bad news: If the other team goes down before you can cause them to retreat, one of the aforementioned Kryptonians will enrage and curbstomp your team quickly.* One group will take on one of the Kryptonians while the other takes on a group of adds on the other side.
Prime Battleground. Remember the Avatars from the Batcave, Fortress of Solitude, and Themyscira raids? They're back, and upgraded to "Prime" Avatars and you have to defeat all of them in succession with various enemy spawns getting in your way. And this is before facing Brainiac himself.
Now it's Booster Gold's turn and the reaction is about the same.
Pro Sabien Sat.7's ownership of SOE's licenses and how completely difficult it has become for a former European player to regain access to their account when the company that now owns it will only give websites in German.
They Wasted A Perfectly Good Character: The Green Arrow of the Bad Future!Metropolis. He was built up as the leader of the resistance against Lex Luthor's dictatorship. But all we get is a screen report of him being captured and the resistance members (after being freed) mentioning going to rescue him. Though from the briefings found, it's implied to be pointless.