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: Ace Online
Demonic Spiders: Bishops, the boogeyman mobs that GMs sometimes threaten to feed misbehaving players to (and specifically warn about in the version 2.2 shop-skill descriptions). They also float around freely in the map called G-ark inside a swarm of missile mobs with itchy trigger fingers (most of the time firing more missiles then you can roll without the I-Gear only Chain Rolling skill). They are "farmed" regularly by some servers.
As of Episode 3, the mobs of the newly-uncovered Pandea maps are nearly all aggressive and farsighted (aggro at 2km, well out of a player's weapon lock-on range), relentless, and have monster levels starting in the 70s (although lv 68+ B-Gears can grind in the ANI Atus map if they have high probability weapons). Players who aren't either daring, skillful, or suicidal only enter Pandea with the help of Stealth Cards.
Feathering and barrel rolling or a damage output that allows you to kill mobs one by one before they can attack you are all that is required to survive in these maps.
Most players would not waste Stealth Cards on Pandea maps, typically saving them for the aforementioned bishops.
Most mobs do not aggro from far away, but some, such as the Bonebat recolours and certain Vatallus attack craft, which have been observed to acquire players as their targets from as far as 1700-2300m. Even then, their attack range is not as far as their lock-on range.
This seems to have been inverted with the newest Pandea maps. Despite being 10-20 levels higher than in the previous maps, most of the mobs die quickly and don't aggro players. You might even go as far as to call it a Breather Level.
Game Breaker: Any Legendary weapon. To this day, there also still exists some in-game bug exploits that rogue players will use to disrupt gameplay (one-hit shots on major in-game bosses, etc).
A-Gears can even be more broken if you know how to do a backshot.
More mundane upgrades that make players cry foul include Advanced Weapon Speed Cards (which make missiles fly faster), Engine Cards◊ (which can improve a player's engine well beyond it's normal abilities, if you get lucky and succeed in enchanting your engine several times), and any weapon which has a ridiculous quick-fire bonus (e15 Bigsmash/Arrow with a grand total of -60% reattack thanks to superior fixes, anyone?).
Nearly everybody has a weapon with at least decent quick-fire or damage bonuses. 5% is not that much of a difference.
Additionally, pierce cards, which make a weapon's damage bypass even more armor than normal.
Radar cards, that let you enhance weapon acquisition range so you can try to hit stuff from even more ludricous distances than normal, may also began annoying players once they get sniped repeatedly by I-gears from 1900m and beyond.
Final skills often add large bonuses compared to previous skill levels. Final siege mode, for instance, can actually triple a gun's rate of fire. Balanced somewhat by the amount of Level Grinding needed to get these skills.
Memetic Mutation: Back in the Space Cowboy Online days, GM Xarfox had generated a meme in which there's light at the end of the tunnel. SCO closed a few months after that.
Play the Game, Skip the Story: How many players actually bother to read the background story about the colonisation of the Phillon Planet, the defection of Arlington, and the sinister machinations of the Shrines?
The fact that you have nothing to do while playing a video game means the story must be incredibly terrible.
That One Sidequest: Ruler of Eopi. You have to kill a high level boss that only spawns every 24 hours. Most players will never even see it.
That's not half of the problem: The map is a bonus map available for winners of a mothership war. In an unbalanced server where one side keeps losing, the map is rarely available. Also, that mission is the last in a string of missions that deposit items that cannot be removed from inventory. Supposedly, it is used to make powerful weapons. In truth, all except one is useless, so that mission is very much required just to get rid of said items.