If you haven't played it in 19 years, its time to!
The great thing about this game is that it gave a new spin on boss fights, the levels had interesting puzzles (try Iceman's level and see), and the music was just fantastic (though, Mega Man 2 had the best tunes).
The game was able to convey a story with very little "cut scenes" or even text and yet you knew what was going on.
The levels were quite large, the use of different abilities came in handy for different levels.
There was no save point (Mega Man anniversary collection had this) but the trial and error really helped the replay value in this game.
What was interesting is the different variety of enemies amongst the different themed stages (ice, fire, electric..etc)
The sound effects such as explosions, of course the music and other background sound like prepellars and such were great.
The graphics were top knotch that early on within the NES.
The game is fairly easy to pick up but to really get a good grasp on the weapons, controls, and platforming elements can really take a bit longer.
The game is not too long if you know what to do, which order to beat the bosses etc.
Other than Mega Man 2 (which is a close second), this is the best game of the series by far.