Metroid was a first in several ways. It was the first arcade/action
game that offered fairly non-linear gameplay. It has its levels with
end bosses, of course, but otherwise it put little restrictions on how
the player explored the game world. Besides, it was the first action
game to feature a female protagonist, and
one of the first games (after Amelie Minuit)
to do so at all.
For more than ten years, Nintendo kept a strict system: There was a new Metroid game for every new platform. There were no remakes, and never two games on the same platform. And each game advanced the storyline. There were three games in direct continuity of the first:
Nintendo first departed from this system when no Metroid game was released for the Nintendo 64.