Accidental Innuendo: There are two planets you visit in Honeyhive Galaxy that are strangely shaped in such a way that they look like... Ahem! Well... if you're male, it's in your pants. Whether this was intentional or not is up for debate.
Awesome Music: The soundtrack is among the most beautiful in all of gaming. Not only does the awesome music have its own page, but every single song on the soundtrack has a listing on that page.
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 two-parter, but still), although he doesn't really get any more difficult to defeat.
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.
Epileptic Trees: Rosalina has been an endless fountain of speculation over the years regarding her nature and her origins. About the former, a very common theory is that she's the Goddess creator the Mario universe. Yes, as in its literal GOD. About the latter, her uncanny, albeit so far unexplained resemblance to Peach makes everyone assume they're both related one way or another. From people who believe she's her lost sister or her future daughter, as well as Mario's... or Luigi's!
Game Breaker: Mastering the game's very loose physics can make the game much easier.
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.
Octoguys are a major pain as well. They're pink chubby Octoombas whose attack strategy is to run away while shooting at you. They're fast and a pain to catch, even when they take a break to catch their breath.
Harsher in Hindsight: In a way, young Rosalina's offhand comment about growing into an old lady becomes somber when one remembers that she becomes The Ageless, and never gets to grow old with her original family.
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!"
The Toad Brigade leader in Grand Finale Galaxy gives us this gem: “Koopas... Boos... Giant, overgrown moles... Whatever! Bring it on!” About seven years later came Captain Toad: Treasure Tracker, A Day in the Limelight for Captain Toad, where he is able to take on Boos with his bright headlamp, and can also catch Pluck Moles to earn prizes. He can also take on enemies like Chargin' Chucks, Hammer Bros, and Magikoopas, which are technically Koopas.
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.
Narm: The drowning sound Luigi makes whenever he falls into quicksand or toxic waste sounds more funny as opposed to Mario's drowning noises.
Nausea Fuel: With the gravity and camera angles (you often appear sideways or upside-down), it can get a bit disorienting. If you get motion sickness easily, play in Deep Dark Galaxy at your own risk.
The Scrappy: Gearmos. Two of them run the trash cleaning missions and will yell at you every time you run out of time. They also appear at the end of the Purple Coin challenges for the Battlerock and Dreadnought Galaxies to chide you if you don't get enough purple coins before reaching them, followed by you losing a life afterwards.
Swimming underwater is a major pain in the ass. 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. It even appears to be this in-universe, as the promotional picture◊ of this power-up shows Mario looking horrified as if to say "STOP THIS CRAZY THING!!!"
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: like the Star Ball and the Sling Pods, it requires remote-perfect precision.
Pull Stars seem to have been conceived just to torture people. It reaches its 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. "Topman Tribe Speed Run" is almost as bad: in one section you have to navigate through a corridor lined by cannons, and if you get hit by a cannonball it is hard to avoid getting hit multiple times while recovering, but that's not the hard part, since it's easy to figure out the proper timing. The hard part is the end of the section, as the pull stars are too far away from the star that warps you to the next section to reach it, even if you drift, and the only way to get over there is to build up enough angular momentum using the last three pull stars, one of which is off the screen half the time. Even then, there is no guarantee that Mario will be pulled in the right direction, making it a Luck-Based Mission. It isn't near as bad on the normal run, since you can take your time, but with the Speedy Comet, if you don't make it on your first or second try, you will run out of time. The Pull Stars are the only part of the level that is all difficult, but most infuriating, if you fail at any point, you have to start the whole level over again, since Prankster Comets remove stage checkpoints.
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. This was likely made into a run because it is next to impossible to not get hit battling the Fiery Dino Piranha.
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 Character: After Kamek blasts Mario into space in the opening, he only appears as recurring mooks after that, with Kamella taking the boss role (and not much of a story role past that.)
Tough Act to Follow: Many people see Galaxy as the gold standard of Mario games and especially the 3D titles, to the point that it currently stands as the best-reviewed video game of all time on GameRankings.note It even managed to outrank the apparently undefeatable The Legend of Zelda: Ocarina of Time! Since then, no Mario title (not even its own sequel, albeit very narrowly) has been able to reach the same level of universal acclaim; in particular, while subsequent 3D titles Super Mario 3D Land and Super Mario 3D World are lauded in their own right, they're widely seen as a step down from both Galaxy games.
Underused Game Mechanic: 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. They don't even return in the Mission-Pack SequelSuper Mario Galaxy 2.
The Woobie: Rosalina in the storybook. As a young girl, she related to an orphaned Luma who was missing it's mother, due to her own Missing Mom, and decided to go on a journey with it to find their mothers among the stars. She spends years in space, and never found the Luma's mother, and eventually realized that her mother had been Dead All Along, and her father and brother had likely followed suit while she was gone. All this happens in her preteen years. She eventually gains a new family by adopting every Luma she could find, but even adult Rosalina gives off a vibe of loneliness.