Here's one courtesy of Medibot and MyNameIsKaz that pinged while they were playing Mario & Luigi: Superstar Saga: The Princess's Secret Slide is an emergency escape hatch to be used when, say, Bowser attacks.
This may explain why he's so limited compared to Wario Land games: he wants the cake and Princess Peach's favors, and he's ready to offer his strength to other characters to show how awesome he is, but if they can do everything else, he won't put the effort.
Remember when you first meet Koopa the Quick in Bob-Omb Battlefield? When you choose to race him for a Power Star, he'll randomly walk off a cliff to take a shortcut to the finish. This seems utterly pointless and random, until you think back to the days of Super Mario Bros.. What was the defining character trait of green-shelled Koopas again? That's right — they walk off a cliff.
One of the ways this game is different to other 3D Mario games is that Mario doesn't have a companion like FLUDD or Luma... or does he? At the beginning of the game, the cameraman Lakitu provides instructions on how to control the camera, not unlike how FLUDD, Luma, or Cappy introduced the player to their central mechanics early in their respective games. Add to that the fact that a free-roaming, controllable camera was one of this game's big claims to fame at the time, and one could argue that Lakitu is indeed a companion like them.
In the original, there are 120 stars to collect. When you collect all of them, Bowser laments at the fact that he missed the 15 secret stars. In the DS remake, there are 150 stars, 30 of which are secret stars. Once you collect all 150, Bowser again laments at the fact that he missed the secret stars. Therefore Bowser managed to find and steal 120 stars the second time around, just like Mario told him in the original game! Too bad for Bowser that there were even more stars that he couldn't find!
That oasis in Shifting Sand Land? It isn't just something you'd expect to find in a desert — it's there to refill your health in a rather hazardous level. The real brilliance is that it's a potential acknowledgement of this Game-Breaker from the developers.
Pannenkoek2012 made a video about how Mario sleeps — one of the places Mario doesn't sleep (and shivers instead) is in a cold course like Cool, Cool Mountain or, oddly enough, Wing Mario Over the Rainbow. Why, then, does Mario fall asleep in Rainbow Ride, when both it and the former are set in the sky? It's likely Wing Mario Over the Rainbow is at an even higher altitude than Rainbow Ride, a case of Shown Their Work on the developers' part.
Alternatively, unlike Wing Mario Over the Rainbow, Rainbow Ride is artificially heated, what with its large house and flamethrowers strewn throughout.
There were fan complaints when "Goomba King" was renamed "Goomboss" for his appearance in the DS remake. However, it's worth noting that Bowser has a piece of dialogue in the former's debut game claiming he begged him to make him a king. This seems to suggest he was originally Goomboss to begin with and only became "Goomba King" through Bowser's use of the Star Rod.
Similarly, King Boo being called Big Boo could have been a translation error... but if you believe all the fan theories of this being "all the trouble you've caused me in the past", it could mean this game took place well before Big Boo became king.
Snowman's Land's name is a play on "no man's land" — which makes sense because no man's land is usually a war term describing a dangerous place that nobody wants to occupy, and that level is one of the hardest in the game.
Here's one for the freaky Endless Stairs. Why does the music sound like it is ever rising? Why can you walk up the stairs for hours and then turn around and walk through the door in a couple of seconds? Because they're looping. The stairs aren't endless, you just get looped back to the start at some point. The music is the same, being a couple of ascending scales looped over, giving the illusion of ever-rising notes. Makes the Staircase a little less scary, now, doesn't it?
When you give the wrong baby penguin to the mother penguin in Cool, Cool Mountain, the mother will say "That's not my baby! She looks nothing like me! Her parents must be worried sick!" That bit of dialogue begs the question: where are the baby penguin's parents, and do they even know where she is?
When you visit the downtown section of Wet-Dry World, you'll find a collection of houses you can't enter and a water-raising switch at the very top. Since no friendly residents can be found and there's only one way to access the switch, it doesn't take a genius to guess what happened to them.
The often cited Wiggler boss fight counts too. In short, you flood the Wiggler's home and then visit it. Rightfully, the Wiggler angrily calls Mario out for that, only to lose in the ensuing battle. Granted that it turned out that the Wiggler was driven mad by the star, but Mario wouldn't know that beforehand; meaning he was perfectly fine with beating up a basically innocent creature.
