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.
Crazy Awesome: Pretty much all the character classes, however the Thief and Merchant deserve special mention. The Thief has an innate ability to dodge anything, and if it's timed right, it'll freeze any attack for about a second, which is well enough time for the Thief to zip from one end of the screen to another and continue mercilessly wailing on whatever's impairing your progress. The Merchant takes this Up to Eleven though. He has a similar ability, only it's a parry, which, if timed right, allows the Merchant to avoid any attack and make money from it. Then they have an ability that literally grabs an enemy and mugs them, holding and repeatedly punching it for money. They can then use this money to either pull potions out of Hammerspace to heal themselves with, or alternatively they can encase their entire hammer with a massive layer of coins, and smash the thing, which is now three or four times bigger than normally, onto anything that's unfortunate enough to stand there.
Game Breaker: Magnum Break for both genders of Swordsman is notorious for this reason. Both are extremely powerful even at lower levels, hits a VERY large area and give generous amounts of invincibility during its usage. It's only drawback is its high SP cost... Which becomes semi-trivial at later levels. What it doesn't kill, it makes go right through the Swordsman class.
While not being totally breaker two Magicians with decently leveled Safety Wall and Thunderstorm borders on this. Two Safety Walls can withstand a lot of hits and give plenty or just enough time for the two spam Thunderstorm cleaning the screen with ease, unless an enemy has a multi hit attack this can be done without any problem even on bosses.
Discount, for Merchants. Yes, it's the basis for the whole class to work, but if you have the reflexes, you can basically invalidate nearly every attack in the game. And those you can't? Spam the Discount button, and see the fountains of cash rise. Cash that can pay for immensely damaging attacks and self-healing, not to mention the passive damage increase as you pile up more and more. Makes up for the otherwise stale skill pool, that's for sure.
Goddamn Bats: Several. The Ants and Scorpions of Stage 2, Knife Skeletons of Stage 3, Orcs and Archer Orcs of Stage 4, Medusas and fishes of Stage 5, Stage 6 has the Tarantulas and the swarm of mushrooms and Stage 7 has the flowers, red flies and the giant red larva.
Moe: Just look at the game's art, that's all. Up to Eleven in the ending artworks of the Female characters.
That One Boss: From Stage 5 onwards the bosses tend to get quite nasty.
Deviace from Stage 5 has some attacks that gets almost the entire screen and comes out rather fast also hurting a lot. When damaged enough he will start throwing entire pillars as javelins, stop time to then massacre the player with a wave of rocks and finally spit a pillar upward which will fall and break in pieces being able to hit the player multiple times with strength enough to kill him/her before they can do anything. If the boss uses the last attack standing right in the middle of the screen pray your character has an invincible move or else you WON'T escape.
Stage 6 boss, Doppelganger, is notably difficult for first time players since he keeps running all around and though his attacks are heavily telegraphed they hit HARD. The Nightmare stampede which comes after his 3-hit combo can do whooping 800~1800 damage (it may hit twice if you get caught in the middle of the stampede), this will bite a nice chunk of your life or downright one-hit kill you.
Baphomet from Stage 7 has very damaging attacks and almost always has two mini sized copies with him, while they don't do terrible damage it's very likely to one break your guard or stun you right in the moment the Baphomet uses his attacks, especially the scythe throw, to make matters worse they can withstand a decent amount of damage.
From Stage EX-1 the Golden Thief Bug which comes tagged with two Demonic Spider which is the only enemy to counter after hit, the boss itself has very dangerous attacks including his Fireball spam which hurts badly and he may cast the Fire Wall right on you locking you in a inferno cage until you die.
Stage EX-2, Drake, has some of the most damaging attacks even for a boss and kill ANY class with his infamous Waterball attack. He also uses his sword in very fast attacks followed by a peg leg kick which is granted to eat more than half of your HP if it catches you.
Osiris of Stage EX-4 is notorious for having a combo of slow down marsh and Breath Weapon which will kill almost anyone not quick enough to use an invincible move or be lucky enough to escape alive.
Pharaoh, boss for Stage EX-5 is infamous for summoning two Pasanas to fight, their attacks still hurt as much as before when you fought one off midway through the stage and also can stun you enough to get caught by one of the many dangerous attacks of the boss itself.