Good Bad Bugs: After you defeat the dragon, the first (and so far the only required) boss of the game, you can leave and re-enter the boss room, and it'll respawn, so you can fight the dragon again. This is great for grinding.
The battle system is very broken and unbalanced. For example:
When you enter the Wimpering Woods, there is a sign in front of you telling you to beware of slimes. However, if you read it, eight slimes will appear out of nowhere and attack you, and it's nigh-impossible to beat that many early in the game. Luckily, you can run from them.
The goldfish. It takes forever to kill and you can't run from it.
Some enemies don't give you a lot of EXP. and are not worth fighting.
Kain is a really bad fighter and most of his special attacks are useless.
The save gems. You need these to save and there are only about 15 or 20 in the entire game so don't waste them.
That One Boss: The Wraith. He's an optional boss, and you don't have to kill him to beat the sidequest he's in,note You can kill the priest instead, which is admittedly easier and has no other effect on the gameplay but if you do decide to to kill him, good luck. Unless you constantly put him to sleep, he'll throw you attacks that weaken you to the point of only dealing 1 HP of damage for every hit, lower your accuracy to the point of missing all the time, or drain most of your party's HP. And all the status effects you are hit with last the entire battle. And the worst part is that he has a SECOND phase where he can grow hands. You're supposed to use the Bottle of Light on him at this point once you weaken him a bit, but if you didn't find it, you're screwed. Even if you do have the Bottle of Light, if your accuracy has been lowered, you can use it and miss the wraith.
