I don't know why people bash this game. It's good, what's wrong with it? I didn't think it was too easy or hard, the difficulty is a good balance of challenge and accessibility. The music isn't quite as good as MM2 or 3's, but it's not bad either. The only thing is that the weapons aren't as good as the weapons from previous games, and you never find a good replacement for the pea shooter, but that's not a huge deal. Just another good MM game.
Personally, I think it's the level design that is lacking. Wave Man's stage is annoying, Stone Man's stage is neverending (way too long), Star Man's stage is even slower-paced than your average underwater stage.
There's also a lot of annoying waiting. You have to wait and pace yourself not to get hit by falling crystals in a lot of pits. You have to wait for a pointless elevator ride at the very beginning of Gyro Man's stage. I already mentioned Star/Stone Man's stages as having pace issues too. I only think that Charge Man's stage is decent, Napalm Man's stage is pretty good, and Gravity Man's stage that is really good. Of course, that part is very opinionated.
And don't get me started on "Dark Man's fortress" bosses. They're irritating to no end, in terms of design. Even Mega Man 6's Mr.X bosses blow those out of the water. The Wily stages are good though.
I may sound like a harsh critic, but Mega Man 5 is my second favorite Mega Man game on the NES, after 3, and I love the entire series. Go figure, huh? I think that it has a lot of originality in its boss design compared to 4. I personally like Napalm Man, Gyro Man (despite a lame pattern) and Gravity Man's designs a lot.
"What kind of moron gets hit by a bus inside a shopping mall?" -Maria Torres, Trauma Team