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.
Demonic Spiders: Hyulkontons. 60,000 HP, at attack which inflicts multiple status ailments to your entire party, and a couple of hefty physical attacks. And guess what: They're level one. Bonus points for being a possible encounter in the first dungeon! A bit of a Guide Dang It, but they're worth the most basic EXP in the game.
Leprechauns. Not only do they prevent you from using your ARMs, but they're level 44 in an area where you'd be lucky to be level 20. To add insult to injury, they have a Confusion ability that targets all party members at once. Sure hope you spent your FP on Up Hp and Confusion protection or you're in for one heck of a fight.
Fan-Preferred Couple: It's pretty much a common knowledge in Japan that Lilika deserves Ashley much more than Marina does.
Game Breaker: Provided you complete a certain sidequest on the second disk, you can locate a hidden vending area called the Black Market. If you can find it, you can buy as many Heal Berries, Big Berries, and Mini Carrots from it as you can carry. It's the only place in the game that allows you to do so.
Don't forget the Game Breaker that's handed to you on a silver platter: Knightblazer. A tremendous boost to all of Ashley's stats and absurdly powerful attacks. Transforming into Knightblazer is basically the game's "I win" button.
Internet Backdraft: We're all open about it on this page, but...don't bring up Brad's sexuality in certain corners of the Internet.
LGBT Fanbase: Brad Evans. Cocytus Ptolomea also has a potent Fan Yay factor.
Don't forget Antenora. Her entire purpose for joining Odessa and getting Vinsfeld to fall in love with her, is to allow herself to be killed by ARMS, so that Vinsfeld would feel the pain of losing someone he loved. This was all Antenora's elaborate secret revenge against Vinsfeld for killing her family years before during the Slayheim War. And it worked flawlessly, devastating Vinsfeld. It's hard to be more of a Magnificant Bitch than that.
That One Boss: Kanon, especially the latter two times you fight her, where Brad is unavailable and thus you don't really have anyone to soak up damage. Belleclaire, the halfway-point monster of Ptolomea's Diablo Pillar, is also highly capable of stopping games cold with its incredibly powerful attacks.
The game's formidable Bonus Boss roster also boasts the Demon King Angolmois, whose 7th Moon attack can full-wipe your party with just one cast (and can be double-cast if he gets the inclination). Then there's series favorite Ragu O'Ragula, who has Angolmois' power and a near-max Speed stat to boot.
That One Puzzle: The first puzzle in the final dungeon confounded players for a decent amount of time. Seems not everyone knows how the days of the weeks got their names or figured out that is what the clues are referencing.