Their key features are a large, interconnected world that the player can traverse rather than individual levels, as well as progressing through this world through the acquisition of power-ups, which allow the player to reach previously inaccessible areas, often also leading to return to a previously visited area in order to get past an obstacle with the newly acquired power.
I would add that there should be a hans crafted map with secret areas. If you don't have these 2 things it doesn't feel like a Metroidvania to me. If any of the stage is randomly generated it's not a MV to me any more. Not saying such games are inferior I just don't classify them the same. For example Steamworld Dig isn't a MV. Neither is Owl boy. It's not enough to have 2d graphics and powerups that let you reach new areas.