Metroid was an NES game made by Nintendo that, while not creating the concept of free exploration in a massive 2D world completely devoid of friendly life, has a major focus on it. There were many places to explore, but the game had major flaws like a lack of a mapping system and many secrets which you have absolutely no business finding unless you were literally blasting everything and checking every lava pit on vertical screens to see if they were fake. The original Japanese release allowed save files, but the American version had a password system added. The game is inferior to Super Metroid, which is considered to be one of the best 2D games ever made in most circles.
Back then, it was a major spoiler that Samus was really a woman, and you only found out if you finished the game really fast. Nowadays, it's common knowledge. Hell, Nintendo's using it for sex appeal now and actually passed a Metroid project to Team Ninja, known for their innovations in jiggle physics.
One of the game's major flaws is that the password system can accept almost anything as a password. With a password generator and proper capitalization, you could make a password that says pretty much anything you want. Because of this, the mystery of Justin Bailey may never be solved.