NEStroid is really easy, but boring indeed. I played only a little bit of it, but what I love about Metroid is the progression, not only in level design, but also in Samus. The abilities, and becoming stronger, is the most fun part in Metroid, to me (along with the environment).
NEStroid doesn't have much abilities, and the environment isn't that interesting. I just play Zero Mission. That was my first Metroid game.