Initially, the game only contained content from the first 6MegaMangamesonNES, leaving fans of Mega Man 7 & 8 feeling left out. This only got worse when enemies from Mega Man 9 were discovered in the game's files, with many worrying that this game would enforce the controversial Fanon Discontinuity of skipping 7 & 8 and going straight to 9, despite WreckingPrograms repeatedly denying this. These worries were finally put to rest with the 1.4 content update, which added a lot of content from Mega Man 7, including re-worked sprites & music tracks to be as close to the SNES original as possible.
Once WreckingPrograms & Luigi1000 begain announcing content from Mega Man 9, some fans were once again worried that the game would enforce Fanon Discontinuity of 8. But this time it got worse because it wasn't just the Dummied Out enemies being officially added in the game; Concrete Man, as well as several enemies, assets, and special weapons were being added. It didn't help that the developers seemed to be very proud of the fact that they were adding content from 9 while glossing over the fact that fans of 8 were being completely left out, and that all fans of 7 had to look forward to were a few new tile sets. However, with the announcement of Flame Sword being added in 1.5, along with plenty of content from Mega Man 8, fans were very happy to see that the game will also be represented. This was even lampshaded in the announcement.
WreckingPrograms: You've waited long enough, now it's time for a surprise reveal! You can look forward to Flame Sword, Bunby Tank DX, new aesthetics and many more features from Mega Man's dive into the 32-Bit era in Version 1.5! You didn't really think we would skip over Mega Man 8, did you?
Ear Worm: The building song will get ingrained to your mind while you're making levels.
Game-Breaker: It was inevitable given how open ended the level creation software is.
Thanks to the first slot allowing infinite ammo, returning game breakers such as Metal Blade and Thunder Beam are now even more deadly since you don't have to worry about losing ammo (not that it is a problem in the first place, as they consume little ammo already). Adding to this, weapons that are powerful but consume lots of ammo also won't have a problem with the infinite ammo slot, allowing to take down foes hard without their weakness getting in the way.
The Perfect Freeze weapon. It shoots a barrage of bullets and has a wide spread, allowing to do rapid damage to enemies. When stopped, the bullets freeze in place for a while, making it useful against collision based robots and stationary minibosses. While it does drain the ammo a lot when holding the shoot button for too long, putting the weapon as your first slot will remove this weakness entirely. With this, you can plow your enemies head on with your P-Freeze with no limitations.
The Mega Ball allows one to perform chains of mid-air bounces with proper timing. When you fire it close to a wall, it results in even easier endless wall climbing, which can often beat the point of thinking up platforming setups and puzzles involving this weapon.
Game-Breaking Bug: Sometimes, crossing boss doors may eliminate the floor collision of the next platforms.
Hype Backlash: Despite the huge media coverage the game received and an immediate, massive player base on launch, several players have noticed that the game is not as well-programmed as it seemed to be, as there are issues like heavy slowdown, glitches, and inaccurate sound mixing. The severe lack of content the game launched with and certain other design choices don't help in the matter as well, and it took three months for the first real content update to launch (which only added a general interference overhaul and two bosses from Mega Man 1), and by then most of the game's active players and press coverage had vanished.
"Deadly Cutsman" by "TehJoos" was a level created full of pixel perfect jumps and mook spam, but still beatable by skilled players. Since then, it's spawned many imitators, such as "The Deadly Cutsman", "Deadly Bombman", and "Diddly Cooksman.".
It's also quite popular to spell out "MEME" using M-Tanks and E-Tanks. Even "Aquatic Ruin", one of the example levels, had this in a hidden room!
Scrappy Weapon: The Needle Cannon was reimagined as clinging on its targets while damaging it slowly. For fans, this creates a problem in that the change negates its rapid fire properties due to the limit of needles present and it can easily leave the player unable to defend himself.
The Woobie: Bubble Man has a design flaw that makes him unable to walk right, which Wily refuses to fix because he finds it funny. If you put Bubble Man in a zone with no water, he'll simply fall to the ground and cry, only occasionally stopping to shoot a single Bubble Lead at Mega Man.