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.
Camera Screw: Mostly averted. The camera is mostly designed to give you the most convenient view when you're on a linear path. The screwing mostly happens when you're in an area that requires exploration.
Everyone Is Jesus in Purgatory: Rosalina is God. Lumas are angels. Mario is Jesus. Peach represents sinners. Bowser is Satan. The game is about Jesus going to save sinners from Hell, causing the universe to be reborn again into a peaceful world where friends and enemies live together in harmony.
Follow the Leader: Since Galaxy first came out, there has been a large influx of gravity-based flash games across the Internet.
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. Kamella spends a lot of her time teleporting, Bouldergeist's hands can draw out the fight if you slam your Bomb Boo against them as opposed to his body, and Kingfin is fought underwater where you have to deal with a bearable, yet finnicky control scheme.
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!"
Nausea Fuel: With the gravity and camera angles (you often appear sideways or upside-down), it can get a bit disorienting.
Nightmare Fuel: ADMIT IT, you were scared when the universe was destroyed. If you understood the Gainax Ending quickly enough for the repercussions to take you by surprise, that is. Also, YMMV anyway.
Kingfin was kinda scary.
Also, right before Bouldergeist explodes, his uvula pulsates, his head shrinks, and his eyes triple in size... and he explodes...
The NPC bees are cute, but take a close-up look at their faces...
The Lumas' cycle of gorging themselves, and then committing suicide by exploding into a new celestial object. They are eventually being reborn somehow, but these guys are happy that they are exploding.
Bears a hilariously disturbing resemblance to a certain kinky fetish.
That gigantic Electric Jellyfish you first see above your head while trying to swim up and around it.
Ruined FOREVER: Bee Mario got this reaction when he was first revealed.
Scrappy Mechanic: 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.
Using the Sling Pods in the Space Junk and Sling Pod Galaxies.
The bubble blowing levels are meant to frustrate you to hell.
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.
Don't forget when the Lumas sacrifice themselves to stop the black hole!
That One Boss: 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, or deal with the trails of fire he leaves behind or the fireballs he keeps spitting. Thankfully, the game doesn't make you do a daredevil run of this one.
Undergrunt Gunner in the Toy Time Galaxy can be incredibly frustrating, especially since defeating him all but requires the use of the Spring Mushroom.
That One Level: Some of the Purple Coin missions, and especially Luigi's Purple Coins.
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. Just Bubble Blast Galaxy. Really, the whole group of the three "Trial Levels" could easily be renamed the "Fucking Ridiculously Hard To Control Levels".
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.
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.
The Woobie: Rosalina. The storybook shows that she became hopelessly sad and homesick after leaving on her adventure with the lost Luma, and she's forced to come to grips with the death of her mother. Even in the present, she gives off the air of a deeply lonely individual, and at this point has outlived not just her mother, but her father and brother as well.