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: Topmaniac, who for some reason gets to guard four stars for being the easiest boss in the game. He also counts as a Recurring Boss (well, to a greater extent than any other boss in the game - even Bowser, who's fought 3 times (the last one's a three-parter, but still), although he doesn't really get any more difficult to defeat.
Breather Level: Beach Bowl Galaxy and Sea Slide Galaxy. Neither have a boss or many enemies, both have a generally relaxed atmosphere and both are based around exploring, side quests and mini games rather than platforming or fighting.
Collecting all of the purple coin stars can be a grueling task. But in the midst of some of the most challenging levels in the game comes "Plunder the Purple Coins" in the Deep Dark Galaxy: a fairly laid back level with only two (easily avoided) enemies, no pitfalls, and some surprisingly simple underwater bits.
Ensemble Darkhorse: Rosalina, a female character in a Mario game who subvertsDamsel in Distress. How refreshing. The reason she's a subversion of it is because she is not physically kidnapped by Bowser, but he renders her ship powerless and nearly kills her "children" by stealing the Power Stars, and she still needs Mario's help before she can do anything about it. Also, turning into/being what may or may not be a Physical God at the end certainly helps.
Game Breaker: Mastering the game's very loose physics can make the game much easier.
Goddamned Bats: Frozen ones, too! They can only be harmed with one hard to aim power-up, and one touch will freeze you for a while. Thankfully, they only show up in one mission.
Goddamned Boss: At least three examples: Kamella, Bouldergiest, and Kingfin.
Hilarious in Hindsight: The game's title was first used in a fan letter published the December 1991 issue of Nintendo Power. The letter in question detailed a fantasy game system which would come bundled with a game titled Super Mario Galaxy, but would be "better known as Super Mario Bros. 24!"
It Was His Sled: The fact that getting all 120 stars allowed one to play as Luigi became common knowledge shortly before the game even came out. Helped by the fact that him being playable in a 3D Mario game was widely rumored and demanded ever since his omission in Super Mario 64.
"U R MR GAY", based on a sheer coincidence someone discovered on the cover of the game. The stars highlighted the letters on the game's logo, spelling out said sentence.
"U R MI ALY", a coincidence found in the logo of Super Mario Galaxy 2. The stars highlighted the letters to form this sentence.
"YA I M R U" , a coincidence found on the logo for Super Mario Galaxy 2, Again, the stars highlighted the letters to from this sentence, but now, it must be read from right to left and from bottom to top.
Swimming underwater is a major pain in the arse. The worst aspect of it is if you overswim your target by just a little bit, it's next to impossible to turn back around without going way out of your way.
The Spring Mushroom power-up. You can't stand still while you're wearing it, the movement is very wobbly, and it requires precise timing in order to do a high jump (and unsurprisingly, the areas in which you find the spring require many such jumps). It doesn't help that it doesn't appear until very late in the game, which means you have to use it on some of the harder levels without having a chance to practice using it in easier levels first.
Ray Surfing in the Loopdeeloop and Loopdeeswoop Galaxies, mostly due to the difficulty of maneuvering the manta ray due to the sensitive motion controls.
Using the Sling Pods in the Space Junk and Sling Pod Galaxies. Even the slightest deviation during aiming can make Mario miss the next Sling Pod and fall into doom unless a Pull Star is close.
The bubble blowing levels, once again because it requires precise aiming.
Pull Stars seem to have been conceived just to torture people. It reaches it's most irritating during the purple coin mission at Ghostly Galaxy, where one must pull themselves past mines and collect coins within an ungodly strict timeline.
Beating Bouldergeist the first time is relatively easy. Beating him again with one hit point for the Daredevil Comet Star achieves controller-breaking levels of frustration.
The Daredevil Comet version of Kamella is even harder. It's pretty easy to get the first two hits on her, but following her up to the crow's nest without getting hit and then dodging the fireballs and Magikoopas to score two more hits on her requires epic levels of patience and persistence. You can spin attack the Magikioopas to get them out of the way for a bit, but considering that Kamella keeps hurling fireballs at you while you try to do this, it's easier said than done.
Fiery Dino Piranha can be incredibly frustrating. Having to time hitting his tail when it's not on fire wouldn't be so bad if you weren't having to chase him/run away from him at the same time. Thankfully, the game doesn't make you do a daredevil run of this one.
The Undergrunt Gunner in the Toy Time Galaxy can be incredibly frustrating, especially since defeating it all but requires the use of the Spring Mushroom.
Some of the Purple Coin missions, and especially Luigi's Purple Coins. The reasons are because of the obstacles present and the need to sort them within a time limit.
Any of the Cosmic Luigi races. Sure, Cosmic Mario's easy enough to beat, since he basically just follows the beaten path, but Cosmic Luigi spams speedrunning tactics like rapid long-jumping and taking extremely obscure shortcuts. Needless to say, if you can beat Cosmic Luigi, consider yourself a master of whatever level you're playing.
Bubble Blast Galaxy, because of the difficulty of guiding the bubble through the electric maze.
The Daredevil Comet mission on Melty Molten Galaxy. Unlike the other Daredevil Missions, this one requires you to play the entire stage, instead of just a boss. A stage mission that's already frustrating.
The trash cleaning missions. You have an extremely limited amount of time to throw bombs in PRECISELY the right spots so the bombs with their annoyingly long timers will blow each other up. Given that in this game, it is nearly impossible to judge where PRECISELY any object you throw will land, you will often miss just one or two of them, at which point you will be yelled at by a robot for not doing it fast enough. Also it takes approximately 10 seconds for one of the bombs to explode, so you really only have 20 seconds.
The difficulty curve of Sweet Sweet Galaxy really got some players with those moving holes.
They Wasted a Perfectly Good Plot: Flying Mario. It's only used in one solitary mission back at the Gateway Galaxy, and for easy access around the Comet Observatory. It's only obtained at the point where you can beat the game so it's largely pointless. Worse is that a spin attack from Flying Mario can actually pull nearby purple coins toward you. If that's not reason enough why they should have included the powerup in other missions, just imagine using it in a boss battle.