If I were to introduce someone into the level design world, the first thing I would show would be, no doubt, a classical in the genre: The first level on the first Super Mario game. It is easy to understand, neat, and everybody played it at some point of their lives!
Platformers were a quite new genre back then, their core mechanics were not engraved on fire in our minds as they are today, and young Shigeru Miyamoto and Takashi Tezuka, the only designers among a group of hardcore developers, came out with a simple way to teach the user what's the game about while avoiding long boring text boxes. After all, if there is something you learn in showcases, is that player's interest in your product is per default very limited: catch their attention at first glance or lose them forever!
What Miyamoto and Tezuka did sounds quite obvious: Just let the user start playing, but set up the scene in a way that makes him confront key items in predefined enlightening ways: For example placing an appealing coin right over a goomba will make the player indirectly learn that if he jumps over this enemy type, the enemy will die. We call this technique "Implicit Tutorials", and despite the concept sounds indeed straightforward, to actually insert the right implicit tutorials into your game, achieving a subtle yet effective outcome, can be harder than you may think.
Back to Reggie, in our game we introduce several mechanics which never appeared in other platformers before.
The principal one is the multi-gravity switch, which consists in turning the gravity upside down with absolute freedom. This allows the player to perform a sort of flight in between changes which we actually encourage: It is fun and it results on a very particular gameplay. But it has a downside too: Its abuse makes the game incredibly boring. You can go from beginning to end of the level just by pressing the same button over and over.
For this reason we introduced an anti-spam to the mechanic: Air friction slows Reggie a little in every on-air gravity switch he performs, losing momentum until running out of power. This variation on the core mechanic does work, and actually improved the physics, since its specific fingerprint helps making the game easy to play but hard to master.
Now... would you paste the text above into an in game text box? Physics are something you should be just experiencing with no need of reading papers about air friction and stuff! It is cristal clear that an implicit tutorial is required.