The idea of increasing perks for Premium players (Increased Cash Cap, etc.). Some state that Premium's current perks are only slightly better than F2P's despite paying for additional content while other believe that they should just stop whining and "become Legendary."
Whether Skill Points should have a greater influence in the game is also a huge one. While they don't necessarily affect progression, there are players who believe that they should play a major part in character progression as they are shown as proof of a player's skills and dedication while others believe that it only promotes elitism.
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.
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.
Red Lantern Guy, freaking, Gardner. In addition to being tanky even for a Red Lantern, he unleashes a powerful area of effect attack that fills up a large portion of the room you'd be fighting him in AND can eat up health almost as quickly as The Flash eats chocolate cake.
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.
Blackest Day. New levels of frustration abound in the Elite Version of this raid and the final boss fight with Black Hand. You have adds whose bodies remain on the field after you kill them and can corrupt players with a nasty Do T that can eventually kill well geared tanks and leave the party open to be K Oed Black Hand's Ao E. If you are stunned your very act of breaking the stun can damage any party members near by and kill them if they have been corrupted. As a grand finale to the frustration, when you get Black Hand down to say 25% health he bubbles himself up and new adds spawn. If you take too long to kill the adds he can regain almost full health! Oh, and you are only allowed to die ONCE in an Elite raid, die again and you are out for the remainder of the fight.
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.