These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Breather Boss: Yes, even SMB has one. Larrie's Lament, the fifth boss, is the first to actually react to your position (you have to get them to jump into the arena's sawblades to win). However, by positioning Meat Boy very close to one of the lower sawblades, they will get killed instead of killing you. This makes them a complete cakewalk, especially considering SMB's normally horrendous difficulty.
Crowning Moment of Awesome: The player gets a small one after finally mastering the complex series of jumping, wall-clinging, and split-second timing to beat That One Level. That feeling of accomplishment is just awesome.
Some may consider The Kid to be this, because most levels are centered around only being able to do one jump.
Tim might also qualify. Immediately accessible on PC via a cheat code, players can abuse Tim's time powers to hold themselves in mid air or go completely invincible until the action button is let go. Coupled with his decent jogging speed and above-average jump height, this makes levels with rising lava levels, scrolling sawblades and homing missiles significantly easier. Or you could use his powers the way they were intended to and fix up any jumping mistakes/bypass difficult back-tracking segments.
Most Annoying Sound: *SPLOCH*...or whatever sound your current character makes when he dies. The screams of the world 5 bosses as well, specially since they ruin the nice music playing in that moment.
The PC version has its fair share of flaws as well, though luckily they're easily fixable. It insists on using a controller, yet a third-party program is required to play it with most controllers. In addition, there's no in-game option to turn off VSync, which may cause a lot of lag and unresponsive controls. Luckily, a mod exists to turn off VSync and cap the framerate at 60 FPS.
That One Achievement: Steam PC version has an "Impossible Boy" achievement. To get it, you'll have to complete the whole Dark World of The Cotton Alley without dying. As if unlocking The Kid wasn't hard enough.