Super Mario Bros. X is a downloadable PC Mario fangame that borrows elements from Super Mario Bros., Super Mario Bros. 2, Super Mario Bros. 3, and Super Mario World, among numerous other games. The game was created by Andrew Spinks of Terraria fame, before he got a purported cease-and-desist letter from Nintendo and left the engine in the hands of the community. The game is most notable for its hugeLevel Editor, which allows the player to make their own levels using the numerous blocks, items, and NPCs available from the aforementioned games.Due to the legal circumstances explained above, we can't really provide a link to the download, but a quick Google search should do the trick.Note: please only include tropes related to the engine itself, and the official episodes released from the old SMBX site. Games made in this engine, should they have enough unique tropes of their own, should get their own page.
Disc One Final Boss: Bowser is merely the boss of World 4, and is fought as in Super Mario Bros. However, he appears again as the Final Boss, and this time you fight him like you did in Super Mario Bros. 3.
Distressed Damsel: Peach is playable, so Daisy takes this role. You rescue her in World 4, about halfway through the game.
Dummied Out: There are a few NPCs that aren't used in the episode at all, such as the regular Propeller Block. They're still in the editor, though.
There are also two NPCs in the "Lakitus are throwing Lakitus" menu that aren't available elsewhere: a blue shell-less Koopa that doesn't work with throw blocks and doesn't have an animation for emerging from a shell (the shell-kicking Koopa was added before regular SMW Koopas) and a ridable airship piece that works like a slower and less useful Clown Car.
Checking the sprite sheet will reveal Peach and Toad had some extra animations, like the former having a unique full-speed running animation and the latter being able to slide down slopes and ride Yoshi. They cannot do either in the final game.
Excuse Plot: Daisy explains that Bowser is recruiting other villains. Said other villains appear very sporadically, and not always on the main path.
Guide Dang It: Many of the secret exits. A lot of them require you to have a specific powerup and keep it for the whole level so you can reach the exit with it.
Infinity–1 Sword: The Hammer Suit. Unlike in Super Mario Bros. 3, you can now throw hammers in two different arcs—high and low. Throwing them at a low trajectory is the default attack, making it easier to aim it than in its origin game. And like in Super Mario Bros. 3, it retains the same gamebreaking lethality, being able to take out virtually every enemy in the game with relative ease.
Infinity+1 Sword: The Lakitu's Shoe, which provides unlimited flight. You need all seven Stars to access it, so that requires completing everything at least up to Dire Dire Rocks, one of the last levels in the game.
Kaizo Trap: "A Short Level" lives up to its name, but it's not quite as short as it first appears. If you take the SMB3-style exit like you've been using for the whole game, you merrily walk into a pit and die. You have to continue past the exit sign, leap over said pit, go down a pipe, and continue on to a SMW-style exit (read: goal post) to actually complete the level. By the way, the springboard in front of said goal post is a red herring—the goal post actually is the end, and perfectly safe to go through.
Also the dungeons, which at first somewhat appropriately evoke Doomy Dooms of Doom (the Dungeon of Pain and the Dungeon of Agony) while later being given increasingly over-the-top and silly names (the Dungeon of Strange Happenings and the Dungeon of Inappropriate Phrases), while simultaneously getting much harder.
Nostalgia Level: There are a lot of them. Most of them are part reproduction and part new content. Of note:
Super Easy Road and Flying Raccoon Zone both have clear references to the first stage of Super Mario Bros. 3.
Retraux: You'll see a lot of this. Especially in the Retroville level, mentioned above.
The Spiny: From two games! Interestingly, only the SMW version comes in an egg form, because SMB 3 Lakitu throws green chasing Spiny eggs (the SMW Lakitu can be set to throw anything, including itself, if you look hard enough).
Super Star: Averted. Redigit was well known for being vehemently against adding this during the game's development, since he thought it would be misused, as in Super Mario Flash's levels. Instead, the Stars act as collectibles, as in Super Mario 64.