The level design of Mario games often seems simple, while gradually and invisibly teaching the player what they should be doing and how they should be doing it. Ideas are introduced in a safe space, and then the safety nets are removed so the player can put those lessons into action before the challenges become more complex.
Nintendo's Shigeru Miyamoto himself walks you through World 1-1 in this video, and you get to see that design philosophy in action. This is possibly one of the most famous levels in the history of video games, and hearing Miyamoto explain the thought process behind the placement of damned near every pixel is a great way to learn a bit about what great level design can achieve.
Now go put those concepts to work in Super Mario Maker.