#4 The Fiend using the red light during matches
It was heavily hinted that the red light during The Fiend's matches was what gave him his powers, but WWE never explicitly came out and said that was the case. In fact, they eventually gave up on the idea of the red light for one reason or another, which hurt the character from a continuity standpoint.
Of course some fans hated the idea of the arena going red during The Fiend's matches, but it also created a surreal reality for the company to play with. It was a distinguishable feature of the characters persona and actually made for some bizarrely awesome moments.
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Again, some might feel this belongs in the same place as The Fiend's hidden weakness, but the light also swerved an aesthetic purpose. It was something that added an extra layer to The Fiend's matches and made the event all the more special for fans to watch. Now, it's gone without even so much as an explanation.