In addition to the squeaky sound, MegaRockMan World 2 also had consistently smaller sprites for everything, whereas the other Game Boy games only shrank enemies where absolutely necessary. Plus the game engine was completely different from all the other games and didn't have the usual polish. Energy fills while the action continues? You can't stop sliding early? You can't jump or slide into boss gates? The screen scrolls past
the first boss gate as you walk up to it? What is this? Oh yeah, and you can't fire a weapon with fewer units of energy than it's supposed to use, which means you can't beat ClashMan entirely with his main weakness! On the other hand, MegaRockMan World 2 didn't have the slowdown issues of the later games, so that's one positive.
Of course then you run into people like myself who HATE MM2. As I'm finding more and more, I'm hoping the "general consensus" on MM2 starts to change to a "Eh, it's got good and bad."
I've been experimenting with weapons in Megaworld The Wily Wars, and I've noticed that most of 2's weapons are major energy guzzlers. Air Shooter and Leaf Shield use 2 units, Crash Bomb uses 4 units, the biggest Atomic Fire uses 10 units--which is pretty much the only useful size, as the medium shot uses 6 units but is usually much less powerful, and the uncharged shots are little better than your standard weapon; and the Time Stopper uses everything left in the bar unless you die first. So you usually need to know which enemies are best taken down by which weapons and when they'll be coming up in order not to waste most of your energy before you actually get to use your weapons to make things easier. And then you have Metal Blade, which works pretty much like Quick Boomerang, except you can fire it in any direction, and maybe a couple more enemies are resistant to it. What kind of stupid balance is that?
Finally, MegaRockMan 2 was also the game that started the tradition of bosses that aren't affected by the regular Buster. So if you run out of your opponent's only weakness, too bad! The fight may as well be over now! But you've got to lose all your lives before you can try again, because the continue point is past where any refills were!