I certainly wouldn't deny that Metroid was a revolutionary game. A game of that size and scope was practically unheard of at the time, plus it was the first game to use passwords. It was also one of the first games to star a female, even if it was intended to be a joke.
These days, the difficulty in the game feels artificial. Having to start at 30 energy every time and fill it back up is just tedious. A lack of a map makes backtracking confusing, especially because a lot of areas look the same. There's no explanation for the items you pick up, so you have to figure them out on your own (I had no idea what the boots item did, so I had to look it up on the Internet before I could continue).