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.
Game Breaker- The 'Overdrive-Stasis' trick. The Overdrive spell gives you up to eight actions in a single turn but drains all your spellcasting resources at the end of it. The Stasis Rune spell locks the caster exactly as he or she is at the moment of casting for the next ten turns or so. If you cast Stasis as the last move during Overdrive, not only do you not lose any JP or WP, but you get to act eight times per turn for the rest of the fight. It gets even worse better when you realize that since it saves all your JP, you can repeat casting Overdrive for free multiple times during the fight, so long as Stasis is always your last move in it.
Good Bad Bugs - Takonomics, named in honor of the man who discovered it: in Koorong, the price of gold goes down as you sell ingots and back up as you buy, but this calculation is (erroneously) made before any actual physical inventory changes hand. You can therefore manipulate the gold market in the shop menu such that when you actually sell your gold, you get more money back. Then you travel to Nelson, where gold ingots are always sold at a fixed price, and repeat until you have all the money you'll ever need.
In addition, the well-loved Overdrive-Stasis trick, even though it seems like it may have been on purpose due to the moves descriptions("Temporarily gain infinite speed" and "freeze time in battle"), is actually a glitch- see Zaraktheus' last post here for an in-depth explanation (the short version: once you use Stasis or the consumable item SnakeOil, this confuses the game into resetting your JP). It can be taken even further with the use of the Shadow Servant spell, which creates a shadow doppelganger that copies any spell or attack you use, effectively doubling the damage dealt in those eight turns.
The Junk Shop glitch, which allows you to get endless free stuff from the Junk Shop in Scrap: just attempt to sell a HyperionBazooka that you don't have and you get seven free items. Perfect for either kitting yourself out with Disc One Nukes or building up enough money to engage in Takonomics.
Ho Yay: From the "Essence of SaGa Frontier"- Ildon and Rastaban are lovers. Asellus and Gina could be considered an item too if you get the Mystic ending.
In spite of one member of the PTB handwaving Asellus and White Rose's relationship as mojo-transfer-by-blood, fandom as a whole ignores this - and no wonder, given Zozma's blatant 'get out of the closet!' speech after the business at the Labyrinth.
Asellus gets a lot of homosexual text with White Rose and Gina, but she and Mesarthim rack up a lot of subtext due to the latter's insistence on being her slave.
Magnificent Bastard: Rastaban. He plots to have Asellus replace Orlouge as the ruler of Fascinaturu and sends Ildon with her to make sure she is safe. The Bastard part comes in when he kidnaps Gina and puts her inside of a Griffon. This bothers Ildon and angers Asellus. However, if Asellus doesn't save her, then no one would know who's really responsible for her kidnapping.
The Scrappy- Slime, who comes from nowhere and goes nowhere. Except in Essence, where he does quite a bit.
Scrappy Level- Riki's Mosperiburg dungeon. From the Magma Slimes to the Mystic Spikes, to the Puzzle Boss fight, it's... unpleasant to say the least.
What Could Have Been: Literally enough to fill a book — we know about a lot of them from "Essence of SaGa Frontier" and "Complete of SaGa Frontier". Fuse was supposed to have his own quest unlockable by completing the others, Asellus' quest was actually supposed to have plot between escaping Fascinaturu and returning to fight Orlouge, several entirely random dungeons and bosses were supposed to have plot significance for one or more of the quests, etc.
Woolseyism- A minor one- Alkaiser's Shin* True-Al-Phoenix becomes Re-Al* Real-Phoenix