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.
Kika and her triple-damage, non-missing, infinite-use Falcon Rune. Throw in Ted (most powerful spellcaster, bar none) and Snowe (gets an ABSURDLY powerful Combination Attack with Lazlo), and you've got a team that simply can't be stopped.
Oh, it gets worse: Grab the duo of Lazlo and Ted, slap a couple high-level magic runes on them (in addition to their default runes), and let them go to town in the Training Hall. Then, take advantage of how the Training Hall works (the enemy levels are based on the highest-level member of your group) to grind the entire combat cast to level 100 in a couple hours (except the guy who runs the Training Hall, of course).
The Hero. Pair him with Ted, spend hours at the Training Room, and train up to level 100, then train the other party members. With the exception of the guy running the place, EVERYONE will be level 100 by the time you're done.
Most Annoying Sound: Playing the minigames can quickly get grating due to the voice acting. They repeat 1-2 lines for every choice you pick before starting the game (ie, which rules, which level of bet, how many potch), and can't be skipped by button-mashing. Rita's is especially grating, as she has a lot of lines of dialogue in the minigame...
Nightmare Fuel: Mao and Nao. There is a genuine sense of dread about who or what has set up shop in the ship's "Forbidden Room". Even when he is revealed, his presence is still pretty creepy—showing up secretly on the bottom level of the ship, talking to nobody, and growing mushrooms/mint in the darkness....
Scrappy Mechanic: Cutting the standard Suikoden party size from six to four is the most hated thing about this game. When you've got several dozen playable characters, only being able to use four at a time is quite bad. In addition to reducing the party combinations you can play with, it also makes unlocking and using all of the Combination Attacks a pain. In particular, the 4-person combo attack by the Razril knights (minus Lazlo and Snowe) can only be used during ship travel by calling them in as a reserve party, since that's the only way to remove Lazlo from the active roster.
Ship travel is also annoying slow, an irritation compounded by the Random Encounter rate being higher than other Suikoden games. You'll be even more thankful than usual when Viki appears.
Army battles in the series have always varied in quality, but the ship battles this time around don't feature any special tactics, interesting maneuvers, or surprises about your enemies' capabilities—you're basically playing rock-paper-scissors against someone missing their first two fingers. Even worse by comparison to Suikoden V, which features both more interesting and larger-scale naval battles, despite those battles being confined to a mere river while Suikoden IV is on the open ocean. In Suikoden IV, the number of ships involved is always in the single digits, turning what are supposed to be major engagements into mere skirmishes.
Viewer Gender Confusion: In-world example: Wendel, a TomboyishAction Girl who declares herself the apprentice of Nico, your ship's lookout. Who, despite his frequently-praised vision and having her constantly hanging around him, thinks she's a guy. And yes, she has quite obvious breasts.