The "chargable weapon" featured as the biggest change to the series in this game is not original. The idea was first implemented in Megaman 2, with the "Atomic Fire" weapon, which could be charged up to deal increasingly more damage (and drain much more weapon energy).
The intro sequence to this game tells Mega Man's entire story to date.
Proto Man only appears once in the entire game - after rescuing Kalinka.
First and only appearance of the Balloon and Wire adaptors.
First Mega Man game on the NES in which Guts Man does not appear (he's a robot leader in 1, a bulldozer fortress boss in 2, and appears in Dr. Light's past creations book in the ending of 3.)