It really was a masterwork in that regard. I really see a lot of the creative genius of that era revolving around working around hardware limitations. Metroid II really did make me rethink what the Game Boy was really capable of back then. How it managed to play so well when the Castlevania games struggled to resemble their NES counterparts really told a pretty telling story in its own right.
Edit: that is a lot of "really"s.
Can we take a moment to appreciate how Metroid II really did the groundwork for what Super Metroid perfected? I don't think SM would have flown to the heights it has had Metroid II not taken the risks it did.
Edit: this wasn't intended as a reply to a comment and should have been it's own comment!