Good Mario Maker tips that will force you into better levels when starting: try to limit yourself to only a few elements per level. 1) The level reads better because the player isn’t overwhelmed trying to figure out what’s coming next and 2) limitations breed creativity; with the low number of elements, you’ll be forced to combine them in interesting ways.
Mario 3 levels have a very solid sense of setting. The pyramid level, the rising water level with the giant fish, the level where go through doors to toggle between small and large enemies. The best levels tell a story, like 5-5, where you wind back and forth in a tower, and finally take a pipe into the clouds. A sense of progression is a good way to make a memorable level.
(Confession: I didn’t make many traditional levels in Mario Maker 1, but I thought about it a decent amount and I think these are solid tips. I plan to make more in MM2!)