One of the most innovative games on the NES. It didn't just start the MG series, but started the whole stealth genre.
It was the first game to employ stealth as a component of the gameplay, it was also one of the first games to employ elements of survival. Everybody who started playing the game just knew that they should hide even more than they should fire, they had to move in shadows and avoid confrontations as much as possible, and stock on ammo and rations for the boss fights to come. It was a whole new way of playing a game, and it was awesome.
The story was one of the most developed in NES games at the time, it was so good that some people continued playing just to know what was going to happen next.
The range of weapons and item was so great and diverse, too. And there were these sequences and boss fights, when you had to use a specific item or weapon to win. The game environment was huge, too, and you had to explore it all to find items & weapons and continue on your mission.
But the game was not without its flaws; there was the horrible English translation, what were they thinking?!! Also the soldiers re spawn every time you enter an area, which makes the game very difficult. And in the version I played you don't get to fight Metal Gear, but to destroy a super computer with explosives instead.
Anyway, I don't think any of these flaws prevent the game from being one of the best games I've ever played on the NES. And it's still one of the classics that started a wonderful series & a whole genre of stealth games.