A simple password system that saves the number of Energy Tanks you have, the introduction of Rush and the totally awesome Proto Man (Known as Breakman at the time), plenty of glitches for added fun, a battle against three Mega Man clones, the return of the Yellow Devil, and the inclusion of all 8 robot masters from Megaman 2 makes this one of the best Mega Man games ever. Good music, refined play control, and something resembling a plot just add to the experience.
My only real qualm is that many of the weapons go unused for most of the game. Sure, the Hard Knuckle blows through walls, topples Top Man, and destroys Gamma, the final boss, but what about the Search Snake or Spark Shock weapons? The latter is only useful to take down one boss. You can actually go through the rest of the game and forget that you have it, as its power is usually more of a hinderance than a help. And the poor Gemini Laser which could have been great becomes almost obsolete because of its slowness. If you like to rotate your weapons you can always find a use, but it's generally more practical to use Shadow Blade and Hard Knuckle the whole time. With a little bit of polish, the weapons could be really cool and useful, but alas, they fall short.