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.
Anti-Climax Boss: The Overlord of Darkness goes down from a single Rider Kick after Shouichi, Ryo, and Hikawa kill the Els of Wind and Earth. Though to be fair, he was in a position were he was vulnerable and couldn't really use most of his powers at the time. And it didn't kill him. He just decides to give up his little conquest against the humans and that's it.
A number of the Lords - despite supposedly being Angels and even making hand-signs for permission to sin - actually display sadistic pleasure in killing their victims. E.g. Corvus Croccio, Apis Mellitus, and the aforementioned Anguis Femineus.
Narm: thanks to a variant of Special Effects Failure. After the black Overlord takes back the powers of Agito, Gills and Another Agito, Shoichi goes through a Heroic BSOD of sorts before literally fighting to reclaim them in episode 46. At the critical moment (see Did You Just Punch Out Cthulhu? above), the black Overlord goes down and the powers he's taken emerge...from his groin.
Nightmare Fuel: The various methods the Unknown use to attack their victims.
Gills himself qualifies as well, especially when you see his helmet come alive.
The Unknowns themselves are very nightmare fuel. From their humanlike appearances to the fact that they're constantly hissing or growling (the only word they seem to say is "Agito"). The BGM that accompanies them onscreen does not help.
Trope Maker/Trope Codifier: Quite a lot for Heisei Kamen Riders. This series was very experimental by introducing four TV Show Riders and three who are the main characters. The Rider who acts as a rival was not fully formed yet but it has its roots in this series as a recurring element. A lot of Heisei era tropes came from this show and many of them were expanded upon in later series. Here's a list:
Movie Riders (Kamen Rider G4 was the first one).
Rivalries between Riders (this series was more subtle about it).