Etrian Odyssey IV: Legends of the Titan / YMMV

Accidental Innuendo: The Landsknecht class has a skill called "Mind Break"^note.
Goddamned Bats: There are Goddamned Vampire Bats in the last leg of the final dungeon-they can't do much in the way of damage, but they can drain HP, have enough health that they can constantly call in another bat just before you can finish the last one off, and to top it all off can team up to perform a skill that drains a massive chunk of TP from everyone in your party when there's three of them in battle. And for maximum irritation, they can also be accompanied by a beetle enemy that can sponge all attacks directed at them! At least both the beetles and bats are weak to electricity so a multi-hitting electric move will get rid of the beetle in a jiffy, but that can still give the bats a turn to call in reinforcements or do their TP-draining move.
Salvaged Gameplay Mechanic: This game removes the much-maligned 1,000-en fee for renaming a character, which served no purpose other than to steeply tax players who changed their mind on a character's name for one reason or another. Now renaming characters is free of charge, and subsequent games follow suit. Sadly, the Origins Collection remakes of the first three games reinstate the fee.
Sequel Difficulty Drop: The game introduces Difficulty Levels to the series: the higher of the two is significantly easier than previous games in the franchise due to much stronger Game Breakers than usual with less drawbacks or skill point locks behind them, though it attempts to make up for it by raising the encounter rate plenty. The addition of small caves that add up to the strata, as well as the expanded execution of the overworld from The Drowned City allows players to better prepare for the more difficult parts (all other games force them to jump directly into the strata themselves, veering into Early Game Hell at the start and Difficulty Spike every time a new stratum is opened).
That One Component: Eerie Scales, which make the powerful TP-restoring Amrita IIIs, are a notorious post-game item drop when it comes to preparing resources for the superboss fights. They're the conditional drop of Moth Lords, only found on the final floor of the Bonus Dungeon; not a terrible start, as while they're statistically very powerful FOEs, they're easy to get a preemptive on and are made helpless by arm binds. However, the "condition" is that Eerie Scales are a Rare Random Drop, necessitating Formaldehydes to get them in any reasonable quantity, which means grinding Horrific Breaths off of Nightmare Rams on a different floor (these are at least guaranteed to drop, but Nightmare Rams are still a problem due to their party-wide petrification) to turn into Formaldehydes. Even with all that effort, it takes three Eerie Scales to make one Amrita III, making this a slow and repetitive process compared to making them from gathering drops in previous games.
Underused Game Mechanic: The Scarlet Pillars overworld introduces the concept of using FOEs as bait for other FOEs, via luring a Dreameater into a position where a Dinogator will attack and eat it. Unfortunately, this is used exactly once to reach an optional maze, and no other overworld FOEs interact with each other. This concept is later revisited in the fifth stratum of Etrian Odyssey V: Beyond the Myth (where a FOE can be lured into another, namely a Clawed Fiend, which in turn attacks it and knocks it down), and is put in greater use there.

YMMV / Etrian Odyssey IV: Legends of the Titan

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