Believe it or not but the first Megaman game wasn't praised and it didn't get very good sales either. Thankfully Capcom believed that it had potential and gave the team permission to do a sequel, as long as it doesn't interrupt their other currently developing titles (if only game development was still like that today). Supposedly they didn't have that much work to do, they already had a bunch of leftover from the last title since the NES cartridges had serious space limitation, but they still polished everything from the ground up. The first thing I noticed was the backgrounds, they are more varied and greatly animated, same effect with the enemies. Now its thing that most people won't notice but back then that it was pretty amazing to look at, one in particular is border line epileptic seizure fodder.
The game also features one of the most badass intro ever created! The story told below, sky view shot, camera slow panning upwards, tense slow beautiful music, suddenly we reach the top, CRACK! Music gets intense, we find Megaman hair aloof in the wind as the title slams in your face "MEGAMAN 2" accompanied by some of the most memorable and epic music ever. Games today lack that, music that marks itself and attaches itself to the game, now-a-days they seek epic orchestras, but they never seem to be memorable. Another epic value that they added was the start of epic big boss battles, fighting against enemies four times your size! Later becoming a staple for EVERYTHING CAPCOM.
They also added new weapons more varied in the form of attack, making them even more fun to play around with. In the first Megaman, personally I simply kept my Buster Canon equipped because there was no advantage with the other weapons. Now in MM2, they gave each weapon its unique attack pattern and in return gave you a gameplay advantage for some situation. They also added these platforms, which you could use to get to higher up places to obtain items or simply make the game much easier; there are a total of 3 different platforms, a floating one, flying one and a climbing one. They also added another component to the game which was the Energy Tanks, what they did was when picked up you could later use them to refill your health bar. Now all of these new components have been a staple to the series so far, so they really got it right the second time around.
I also need to gush about the Metal Blade. The Metal Blade is the most epic power up ever created. You get from defeating MetalMan, which is a real easy by the way, and then you get this awesome powerup. Complained about not being able to shoot up? Then get the frikin Metal Blade! It shoots in eight directions! It even somehow breaks the whole robot master weakness circle, simply because it so versatile and powerful! I love the Metal Blade, bow to its awesome power!
Only negative output I have about the new features is that, the game isn't designed to have all of these, it still have level designs from the games draft and it doesn't exactly draws the full potential of the new features. In correlation it only makes the game easier, even with the supposed "difficult mode" MM2 is one of the easiest of the series. But because it is the easiest I suppose it would be the good one to for a newbie to start with, it still lacks some features from the future games but it holds up incredibly, most say this is the best Megaman game ever, sales do to, MM2 was the highest selling Megaman game ever and it still stands today.