In an interview, Shigeru Miyamoto said about designing the Donkey Kong arcade game:
[A] fun game should always be easy to understand — you should be able to take one look at it and know what you have to do straight away. It should be so well constructed that you can tell at a glance what your goal is and, even if you don’t succeed, you’ll blame yourself rather than the game. Moreover, the people standing around watching the game have also got to be able to enjoy it.
Super Mario Maker allows designing all kinds of levels. You can design a one-screen puzzle level. You can design a long level that goes up and down and up and down the two screens of vertical height available. You can design a very difficult level that throws enemies at the player every step of the way. And you can also design levels that conform to the standards Nintendo follows in their Super Mario games.
The Mario Level Design blog is intended to help with the latter: creating levels that more closely resemble Nintendo’s Super Mario levels.