Broken Base: The game's soundtrack has divided a lot of players. Some people feel like this is Yuzo Koshiro's weakest and most boring score, while others will tell you he aimed for an ambitious and unique attempt to emulate an orchestra.
Game-Breaker: All of the infinite weapons in general, but especially the Infinite Omega Sword. It even sounds gamebreaking.
Low-Level Run: This is easy to do in Beyond Oasis since your character's level is determined by the number of hearts he collects (which also, predictably, increase maximum HP), rather than EXP gained by kills. Furthermore, hearts are rigged to only drop from enemies if you're getting your ass kicked, when you would logically need them. Thus, the game's difficulty rather ingeniously scales itself depending on your skill... or, you can simply go macho and ignore hearts altogether.
The secret 'Efreet racetrack' minigame in Beyond. Getting under a certain time gets you an early Game-Breaker in the form of an infinite Fire Bow. To get under that certain time, you need to master controlling Efreet's charge attack, which is fiddly at best and infuriatingly uncontrollable at worst.
The jumping puzzles needed to unlock infinite bombs is almost as bad, considering this game was really not designed for the kind of precision platforming you need in this sidequest.
