what SOTN really added to the formula was rpg elements like leveling up, item drops (esp. rare item drops) and equipment customizationSuper Metroid is one of the greatest and most influential games of all time. in fact I claim the term "metroidvania" isn't accurate, because those mechanics really come straight from Metroid. Castlevania didn't have them until Symphony of the Night (at which point it got them from Metroid)
the term is overused but i think a true metroidvania needs both the exploration and ARPG elements