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.
Refia is arguably the most popular of the four DS protagonists, only rivalled by Luneth.
Demonic Spiders: Every enemy that splits when it takes physical damage from a non-katana. They can quickly prove overwhelming if you're not prepared for them.
These foes are easier in the DS version, in which these enemies only split on their turns if they're hit with a non-katana instead of immediately after they're hit, and only three of them can be out at a time, making it easier to kill them with a multi-target spell, or with the Darkness Wave of a Dark Knight.
The Geomancer's Terrain skill, also in the remake. At high Job Levels it does damage nearly equal to (or in some cases, greater) than a Summoner and can be used every single turn for free. Even early on it is a great alternative to using a Black Mage, since even at low Job Levels it usually outperforms the second-tier elemental spells. The only downside is that the attack chosen is random (and a few of them are instant-death attacks, which means a wasted turn on bosses), but nearly all of them do the same amount of damage to most enemies. The exception is the rare attack Shadowflare, which will do thousands of damage even at Job Level 1 and will reach 9999 later on. Three Geomancers plus a White Mage / Devout can rip through most of the game with incredible ease, especially if you grind Job Levels early on.
Notably, the chance for Shadowflare increases drastically in the Bonus Dungeon Eureka, which contains the best weapons in the game.
Good Bad Bugs: A bug involving the inventory in the Famicom version allowed the player to acquire the very powerful Onion Equipment at the start of the game. (It takes a huge amount of time to set up, however.) Another one in the DS version allows players to duplicate any consumable item in the game, up to and including the rare and precious Phoenix Downs.
Which doesn't make sense. How can the west feel apathy if we never played it? It's just that a Wonderswan FFIII remake never happened. That's all. Square was too lazy to put it on GBA during the Finest Fantasy for Advance campaign.
Nightmare Fuel: The Nepto Dragon's design in the original Famicom version is...
Seinfeld Is Unfunny: The class system this game introduced to the series has been done better in so many other games by the time it was finally localized it doesn't reach the standards of even V. Never the less, several gamers found it to be a refreshing break from the immense complexity of other titles in the series.