Devil May Cry 2 went through several changes for each release due to the initial development time in PS2 release. There are gameplay differences on each DMC2 release and some of them still require further testing. The difficulty changes are only affected on enemy behaviors and stats. It does not affect characters stats or Shop items like how DMC1 does.
Defects in Offense Heart and Quick Heart - Offense Heart had no effect on the attack power increase. Instead there was an issue with Quick Heart which raises speed and agility. Meaning, NTSC version had an ability where you conserve DT by skipping Support Hearts all together. Therefore, PAL fixed it.
Soft reset - This is usable by simultaneously pressing L1, L2, R1, R2, select, start in PS2.
Trish's Kick 13, Magma Drive, Jump Kick effects - PS2 have heart attribute bug on Trish. Flame effects are attached regardless of elemental heart using these techniques with Frost + DT makes it feel a little unnatural.
Trish's Magma Drive - PS2 is the only version that can charge this move (Need testing to confirm).
Trish sideways flip and backflip - Works in NTSC-J only. The rest of release Trish can only do front roll.
Dante: Laevateinn, Dooms Day, Lucia: Divine Anger technique can not be used continuously - These moves can be used infinitely by timing △ + ▢ button. It will keep going forever, HD collection and PAL fixed this. Video demo
Jokatgulm fight, bug in doomsday - PS2 have a bug that hitting Jokatgulm with doomsday causes you to not damage to it's main body anymore. (Need to ruplicate this)
The end of the title intro became common to Dante and Lucia - Not gameplay related but, the opening intro ends with the effect that “the character zooms out while taking a pose, it becomes the title logo silhouette as it is”. In PS2, a character taking a pose Dante if it is Dante, Lucia if it is Lucia, but both have become Dante in HD Collection.
Status Menu - Menu controls are different. In PS2, Status Menu is select. HD Collection made it start instead.
Final Noctpteran hatches can not be destroyed - When killing Noctpteran, It lays two eggs after falling to the ground, those eggs can not be destroyed. Additionally, the eggs in HD Collection behaves differently. In PS2, you are able to destroy with a shotgun or by any means possible.
The sound of gunfire disappears - When fighting against Nefasturris with Trish. The laser, round trip and hand gun's firing overlap, the sound of shooting gets cut off.
Graphical bugs - The graphics of the following attacks are disordered:
Missing sound effects - There are plenty of sound effects from PS2 that are either replaced or disturbed such as Despair Embodied's shape shift. Dante's majin form sound effects are replaced with human dante.
Boomstick Glitch - This is very recent on PS4/XboxOne/PC port. A glitch where when you shoot the shotgun no matter where you areaiming the shotgun blast hits all eneimes on screen. It is not replicable.
Flame Heart Glitch - A glitch that only works in PS4/XboxOne/PC port. With Dante, if you hit Round Trip at certain environment or against the enemy in specific spot, everyone else will get hit. It will also break any breakable object nearby causing his DT gauge drained much more than usual. Lucia's Aerial Cross for some reason generates unusual massive damage effects. This works against Orangguerra, Noctpteran and Arius-Argosax.
HD Collection fixed clipping in M15/M11 which is used as Skip that is no longer possible to perform.
Changing HD Collection 2018 text to JP text does save a little time over ENG.
Trish shares the same movement properties as Dante and amount of damage using kicks does not scale with the weapon.
Holy Water in this game acts as a drop instead of item usage. It appears as a part of green orb drops with a probability of 256. It deals 5,000 damage to the surrounding enemies and 500 against boss enemies.
This will only work in PAL and HD Collection, if you change its language to European country. There is a code that you will get to use after you beat MUST DIE mode. Keep in mind that this must be done separately for both Dante and Lucia campaigns.