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.
Special mention goes to Bird Against the Storm, the music that plays in the Lair of the Evil Dragon. While most of the game's tunes have a sort of upbeat techno feel, this one pulls out the heavy guitars out of nowhere. It does a good job of getting you pumped for the fight against Invoke.
Game Breaker: Super Puzzles, Prime EXP holy waters (6x the EXP for 10 hours, anyone?), and most fashion equipment of all things, with the latter being most prominent in PvP.
Spoony Bard: Strangely enough, Temple Knights seem to become this later on, when everybody has found a way to effectively not die, and tanks become obsolete. Thankfully, they now have the option to become Terror Knights.
Crutch Character: Before then, knights that use a shield and have decent armor are very hard to kill. They are the only class that can effectively take abuse from every monster on the map when the circumstances are right.
However, the high-level Gunslingers have a reputation for being arrogant, obnoxious, and elitist.
Many people feel this way about high-level pvpers in general, but that's true of just about every MMORPG in existence.
That One Boss: All the scenario dungeon bosses and the King Goblin and Selkie for newer players.
That One Sidequest: All the Selkie quests. Especially the last one, which combines That One Boss with a low rate of Randomly Drops and makes you do it 10 times. Per quest. DotNuri is a nightmare too, especially considering you need to do each stage twenty times to get each bonus. Many players find stage one near impossible.