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: Healing or buffing any ally earns the character casting the spell 30 experience. Every 100 experience levels you up. Leveling-up refills your HP and MP. The exploit here should be obvious.
Narm: Every single chapter, there will be a horrifically voiced and narm-tastic friendship speech, and sometimes twice. Every. Single. Chapter.
The stuff they say during battles. Theo's "I won't let my brother down" when at half HP isn't bad on his own, however, when Alph (said brother) is already dead, it turns into this.
But Lady Claire really takes the cake. When you fight her, at the beginning of her turn, she'll say "Please, be careful" and then she proceeds walking up to your team and wipe half of it out with one single attack! The worst part? Before she attacks you, she says "Take this... Please?".
The Scrappy: You have no right to declare anything scrappy until you've seen a ganguro valley-girl angel that constantly giggles at your misfortunes, ideals, and sacrifices, and refuses to just shut the hell up. Scrappy, thy name is Priel.
That One Boss: In the New Game+. You might think that the battles against Vanessa, Heath or the three witches Claire, Vivi and Mavi would be a breeze now that you are at level 50 or moreso, Wrong. If you made the mistake of level grinding them more than the rest of your party, you are going to pay the consequences, let's say... having to fight a Lv 70 Heath with a Lv 50 Alph. Cue Curb-Stomp Battle. What is worse, it's awfully easy for it to happen because you can earn exp thrice as fast just by buffing and healing over attacking. No points for guessing which characters can do that.
The battle versus the Witch of Immolation in Jeidath. First off, parts of the ground are Thunder element, which powers up the already-powerful Gul and Gal. Second, the fire Hikopins know Heatwave, a powerful fire attack that can hit up to 5 characters and is ranged. Third, the level is split two ways so you can send your party one way or send one group one way and the other group the other. If a majority of enemies get to one party, everyone is screwed. Not only that, both your healers are Water-element, and are therefore weak to Fire, and their bad Resist/Guard and Movement stats make them just lag behind, unable to get to the front where the more damaged members are likely to be. Oh, and your healers are horribly slow, going after pretty much everybody. (The third one is Light-element and has better stats overall, but she is still just as likely to get burned if she gets ganged up on.) While this battle can be beaten easily with a bit of level grinding, luck and strategy, it certainly trips up players that get to it the first time.
Alph and Theo are dragons. And that's all exploration this is given.
About their history before becoming Garden Children? Yeah. But, the fact that Alph is a dragon/Rym is still important, as it gives him an automatic soul bond with Lucia (sparking the main plot), and parallels how dragons and witches worked together to defeat Zehaal in the world's history.
Heath, usually a Reasonable Authority Figure, is marked for much of the game by his unreasoning hatred of all Witches, even trying to kill his favorite pupil for siding with one. Why? No reason. He was Good All Along.
This one is actually explained. His childhood friend Iris was "killed" by Witches. OF course, that's just a cover-up, she was actually turned into an artificial Witch herself. The first point explains his racism, the second why he finally outs himself as a defector.