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.
Game Breaker: This game has far too many. It's a pretty easy game in the first place, so the following examples just make it a cakewalk.
KOS-MOS has four skills that would be useful for anybody in this game: Mode A7, which increases her attack by 25%; a Move Again skill; and S-Chain, which would lower the defense of opponents by 25%; these skills costs are expensive to cast, but she has quite a large mana pool. Her real gamebreaking skill in this game is that her MP refills by attacking opponents by 5% of damage done. After attacking (assuming you used the 60 MP attack again skill) use Mode A7 and attack the opponent again. Not to mention that KOS-MOS has the highest attack and defence power in the game, meaning she's usually out on the front lines getting the majority of your team's attacks in, and therefore tends to level up quicker than your other characters. She can also 3-hit Branch. AND, whenever she gets hurt, she has a self-heal spell at her disposal too...
Armor King is like a miniature version of KOS-MOS, since he has Move Again Anyone and some other useful spells, BUT with a tiny mana pool, less than half of KOS-MOS. He also has MP refills by attacking, but he does perhaps half as much damage as KOS-MOS.
There's also an exploit involving MP Pots and the +10 AP spell. Basically, when you get 2 or more characters who can cast this spell in one stage, you can give them an infinite turn loop, letting them go again and again until the mana pots run out. But the enemies are usually all dead before then.
Stahn and Rutee, Gilgamesh, and Kai and Judas, are basically the ultimate tanks. They gain 5% mana from damage inflicted on them, and they can cast healing.
Stahn and Ruhtee can heal ALL allies for moderate cost. So, they're sort of unkillable. They eventually gain the ability to 2-hit branch with almost any 2 attacks combined. (with a gain items easier type item equipped) This means they can get Health and Mana bonuses while doing this trick. OR, if the enemy is really strong, they can try to stick to the attacks which he is elementally weak against. Their Limit Break automatically does a 1-hit branch too. You can perform 2 Limit Break on one opponent too...
Judas's healing spell also renders him invisible, which means he can't even be attacked by the enemy, including bosses.
Abusing the air combo system by letting enemies drop. This will net you huge amounts of Branch Ups (extra attacks) and Damage Ups, which will greatly increase your combat damage, AND will also give your characters huge EXP bonuses. Item Type characters will also obtain huge amounts of top quality items, just make sure that they have the 'gain items more easily' skill or item on them. Branch Up in 2 attacks is referred to as 2-hit Branching. Knowing which attacks begin the combo count even on standing enemies is crucial to this.
For Guy and Sho, it's That One Attack: Smoke Bomb(Strong). That attack alone garners up to 9 hits and all you need to do is just use it twice for 2 hit branch, since it does combos for 9 hits off the ground. Oh, and for added insult, enter a battle with a full super, then use a smoke bomb followed by their Bushin Musou Renge right away, and then smoke bomb the enemy to oblivion. You're guaranteed to pull off TWOlimit breaks on just one opponent; assuming the opponent is strong enough to survive that long anyhow. Guy's 9 hit punch is also useful for the 2nd hit, since it's slightly stronger than smoke bomb.
Kurino and Sabine do a 2-hit Branch with their spinning attack followed by spear attack. Their healing spell also renders them invisible.
Early game, as soon as you can level up Bruce and Regina to level 8, have them attack enemies at close range and just use Regina's submachine gun move twice for a 2 branch. Later on though, this falls prey to Disc One Nuke factor, since it's their weakest attack. Still useful for gaining a bucketload of top quality items though.
After Chapter 9 (Ryu and Ken meet Akuma), you get to play the gambling game, where you can easily double your money each round. This can be abused by saving game in-battle just before completing Chapter 9. It also helps to have hoarded up tons of gold up to that point. If you can double your money a few times, you basically end up with more money than you would normally earn throughout the entire game, which will let you buy all the items you could ever use. Note: Sylphie's Dress kicks ass.
Shipping Goggles: An in-game example: the heroes believe that Saya is Reiji's ex and the two went through a really bad break-up, despite him outright telling them in the previous chapter that Saya was responsible for the murder of his parents. Then again, all of that Foe Yay didn't exactly clear up much.'
That One Level: The level that everyone was making some incredibly lame punsbecause of some spell or something. You get tossed in a level in which you're facing Red Arremer, the Genji clan, the Lord Raptor-Black Bravo-Amazona team, and a ton of mooks. While you do get Sylphie as a back up, the only decent characters you have at your disposal at that point (unless you've been Level Grinding) would be Morrigan & Lilith, Gilgamesh & Kai, and Bruce & Regina. And you're surrounded by all of your enemies while you do this. Unless you abuse the fact that 3 characters have the +10 AP spell, which let's you slaughter the stage with ease. The main problem with that stage is that your characters have low attack power, so they take a LONG TIME to kill all the enemies.
Next level is harder. There is Shadoloo head rampaging the battlefield, he has insane attack power, and you cannot damage him at all until you defeat another enemy, which is not on the field at the beginning. And there is Saya with bodyguards, all of the trio can inflict poison.