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.
Serial Numbers Filed Off: The first game is in many ways a retread of the earlier Toaplan shmup Twin Cobra. The homing missiles also seem to have taken their trajectory routines from Toaplan's Slap Fight/ALCON. On top of that, "Gallantry", the song for stages 1 and 4, shares a phrase with "Mystic Green" from Toaplan's Hellfire, and "Lightning War" (2 and 7) sounds like an extended version of the second half of "Against the Attack", the st. 2 song of Flying/Sky Shark by...Toaplan, again. The first Raiden was, basically, a doujin of Toaplan precepts.
They Changed It, Now It Sucks: The Proton Laser in III. Instead of being purple and twirling all over the damn place, it's green and only swings from left to right depending on the player's movement. (At least it dependably got foes behind that big vessel...) The outcry was such that in Raiden IV, you get to choose between the Plasma Laser and a triple-beam (and purple) version of the Proton Laser.
On the other hand, the way the Plasma Laser coils in IV is dissimilar to previous editions. The suspicion among some fans is that Seibu had simply lost the original algorithms, and only created the Proton Laser the way they did because of that.