Broken Base: Happens a lot of times when it comes to gameplay and the forums. Mention your opinion on a single unit in the game and watch people debate and disagree over whether or not a unit is over-powered, too weak to use, or perfectly balanced.
In terms of gameplay, MCA and special attacks are definite base breakers: Is MCA cheating or is it showing your skill and overwhelming a player? When's the perfect time to use specials: When a player is weak? While they're strong? Why the hell did you just use it when I only had one HP?
Kill Farms, or artificially boosting up a kill count by diving matches. While the official stance by the original game moderators was that it was against the rules and, if caught, they would be punished, a new set of mods came in and, without their input, left many players confused and arguing the stance during a weekend event. Opponents of kill farms call it flagrant diving and that everyone who does so should be punished for doing it, especially since the divers clog up the servers with these rooms and leave others who want to play normally in the dust. Some others do tend to agree, but find it a necessary evil as many players can't get 10-15 kills a round and don't have all weekend to do so.
Goddamned Funnels: The Bits/Funnels/Fin Funnels/Gunbarrels/DRAGOONs will easily swarm you if a user of one gets you in the line of fire. They may not cause too much damage, but they're annoying as can be.
It's Popular, Now It Sucks!: Easily the fate of event-earned Mobile Suits - players will obtain suits, level them up all the way to Over Custom 5-6, then unleash them on casual players in PVP mode. Some of its worst offenders were Shin Matsunaga's Zaku II (one of the event units with a dash Bullet Zero attack), 00 Raiser (Rank SS unit with ungodly power) and Astray Blue Frame 2nd Revise (another event unit with a dash Bullet Zero attack), 00 Gundam Seven Swords and Wing (EW), dealing high damage with all weapons, The Stargazer Gundam (FAAAAAAST) and the new 00 Raiser, the 00 Quan[T]
"Stop Having Fun" Guys: Like many other games, it's not uncommon to see someone enforce rules to "ban" certain units from PVP matches. For instance, some players refuse units with the Counterattack skill (which allows a player to fight back when they're attacked), the God Gundam and even the Neue Ziel.
Most bans were typically in melee matches. Counterattack gave units super armor to melee hit stun, making them unfair and just less fun to fight.
That One Mobile Suit: There's always certain mecha that will cause you to rage hard when you encounter them. Some of those include:
The Psyco Gundam of the "Destroy Three Psyco Gundams" mission. In harder levels, they're able to use their special abilities an infinite amount of times, thus leading to some instances where you're about to kill the last one and he'll use the special just as time runs out.
The Acguy Repair of the "Destroy the Apsalus II" mission. It's already annoying as it is The Medic, but it has the added annoyance of being one of the few units who block out Auto Lock-On, which means while you're trying to hit him, he's busy bringing the Apsalus back to full health!
The Gunbarrel Dagger. The only C-Rank Mobile Suit with bit attacks. Which means 9 times out of 10, entering an all-C battle will mean seeing this thing coming at you.
Even PVP isn't safe, as there is one Gundam that to this day that can cause people to quit on sight or kick the person using it. That unit is Nu Gundam HWS (Heavy Weapons System), an amped up version of the Nu Gundam. It lacks Nu Gundam's beam-proof barrier, but instead gains a flashlight beam gun, homing missiles, and funnels that when not attacking, will actually protect the user from ranged attacks. And then it's skills, which not only increase the reload rate of its weapons, but also increases attack, movement speed, and puts enemies on radar. As if that isn't enough, it can also purge its armor to become faster and remain arguably just as dangerous.
Now there's the 00 Raiser, an improved version of the 00 Gundam. Both GN Sword II and GN Sword III forms uses an ungodly amount of blades and beam rifles, but its real skill is its "True Innovator (00 Raiser)" skill, which allows the player to "Quantize" (meaning that they're cloaked and gives the illusion of teleporting in game) and this happens whenever you're attacked.
God Gundam has this effect with some playersnote The weaker players or the more obnoxious ones it seems.. Bring it into a melee room and someone will instantly say "no god", kick you, or switch to Neue Zeil (which is three times bigger) in response. People will actually prefer to fight 00 Raiser or the S rank form of Deathscythe Hell, which can turn invisible instead of this one.
God Gundam and Neue Ziel were Immune to Flinching, meaning in a melee match they couldn't be hit stunned. As soon as you got a hit in, they would just start comboing you, stunning you until you died or got knocked down, and there was nothing you could do about it (unless you also had a unit with super armor, leading you two to both be meleeing at the same time until one of you died, or you had Sandrock Kai EW which inflicted stun with its melee).
And for that instance, the Neue Ziel. It is currently one of the few Rock-type S-Rank Mobile Armors in the game. The unit has an amazingly fast boost and, as a Rock-type, specializes in melee. Combine that with it having incredible defenses (as befits a Mobile Armor) and an I-Field (meaning beam shots won't hurt it as much) and you got a recipe for disaster...
In relation to that, the SR-rank Missile Launcher type. While it loses the original's tankiness by becoming a Paper-type Mobile Armor, it dumps most of its melee attacks for ranged attacks, including a claw attack and quick-reloading beam cannons and missiles. As well, it keeps both of its normal S-rank counterpart's skills, so it's still deadly.
On a related note is the BS-rank GP02A Physalisnote The key unit to building the S-rank Neue Ziel. It may only be a Scissor-type with a beam saber, vulcans, and a shield bash for its weapons, but it has a Nuke for a ranged special attack, and backed by an ungodly high special stat. Add the fact that it also has the Ziel's signature skill Spirit of Zeonnote When the unit's health drops below 50%, the unit's defense, agility, and special attack damage are increased and the team's attack is greatly increased., and with an equipped "Awaken Special Ability"note Increases special attack damage when HP reaches 30% skill, and it's a guaranteed One-Hit Kill, to almost every MS below its rank, and can even eat through the Health pool of S-ranks with one shot. The only thing keeping it from reaching Game-Breaker levels is the HP requirement to activate both aforementioned skills.
Wing Zero is the bane of snipers. Since it is a Scissors-type with a long range attack, so it easily counters Paper type snipers.
The two December 2013 Rare Capsule units are turning into this. The Wing (EW) has a ridiculous damage weapon in Bird Mode. The 00 Gundam Seven Swords gains a huge attack bonus, combined with the normal 00 Gundam's 00 System skill, which increases damage a lot already. The result was a super-powerful melee combo on a similar damage to the God Gundam's combo attack.
They Changed It, Now It Sucks!: The common cry concerning the Web Gashapon, as many players will bemoan the appearance of units that would appear by the next update, lame C-Rank units and bad extra prizes. All of this is mostly due to the fact that they already HAVE these units and a lot of them are more geared towards the new players.
The sad fate for many suits with the G-Generation/G-Revolution update. Thanks to numerous balances and the removal of the Custom/Over-Custom system, many units have lost their relevance.