I think it's more so the additions of Rush, Protoman, sliding, and the Gamma boss fight.
Personally I just want to know why 2 gets so much praise. It added 2 stages and E-Tanks.
People hate the Magnet Beam (which you're dumb if you do), but never complain about how each Wily level forces those types of sections on you in spades on top of Bosses that are only damageable by a certain weapon. Not to mention if you get a Game Over you lose all your E-Tanks (MM&B is less punishing than that). Run out of Crash Bombers on Wily 4? Good luck killing yourself or farming at random for ammo.
2 just comes down to solid level design, weapons and music for me. Even without spammy-mic-OPBlades, I adore doing runs with all the weapons in that game, it is the one and only instance where I can say with certainty that I love every single weapon in a Mega Man game.
The bit with E-tanks never bothered me. Plus, 2's "normal" mode is pretty much everyone's recommended starting point with Megaman, which is probably why it's so popular. Dispicably OP Mega Buster aside, it's still plenty challenging for someone who has never played a Megaman Game in their life.
All the Classic Megaman games have their own unique high and low points, I guess it's just why the series has so much charm and got away with having 6 games on NES well into the SNES life cycle.
And yeah the Sentry Turrets boss can eat a massive, spotted metalic.... hot dog.
I do agree because the Doc Robots make the game unfun in my eyes and I hate how people just say "get gud" when it's kinda hard to when you have to learn weaknesses for robots who have MM2 powerups and how some have some crazy patterns and Protoman pls.
Even then, the only MM game that upset me was Megaman & Bass by it's just BS level design and be warned. I played the GBA version which is the worst version as a youngster. I hate the setup, how neutered Rock was compared to Bass.(He doesn't have his Super Adaptor but Bass does.)
That game is actually no good.
It's the only Classic Megaman Game I have yet to play for that very reason.
Like holy crap, who promoted a chimpanzee to the lead level and gameplay designer for that game?