I just came across The Linear RPG by Sophie Houlden, and thought it was a brilliant example of focused gameplay design. Usually we don't post about other games, but I think this is a good way to learn about the core mechanics of RPG games.
At the heart of most role-playing games are two forms of progression: leveling up, and continuing the story. The only real decision you ever have to make is whether to continue grinding levels, or try to get to the next safe area. While different RPGs spice things up with various lower-level mechanics and fluffy choices (do you want to kill enemies with swords or spears?), they all have the same underlying structure, which is elegantly illustrated in this game.
I really enjoyed The Linear RPG because it's nothing but the distilled essence of the genre, serving as a powerful demonstration of why leveling up and story progression are so addictive. Leveling up just adds hitpoints and the story makes no sense, but its simplicity and self-awareness makes it a worthwhile experience in its own right.