Making bad levels is easy. Making good levels is really hard. The player should be rewarded when they do something extra. For example, if they bring a koopa shell a section even though it might wake up in there hands and hit them, they should have a 1-up waiting for them. Also, make your level look nice: Replace some ground tiles in your cave level with the blocks that can only be destroyed by tough enimies. Place bits of ice in your snow level. Mix and match semi-solid platforms. Don't build your level on 1 ground tile or make the ground float.
But of course, I'm not omniscient. There are plenty of ways to make your levels better, and if you look out for what makes you like a level you play, you'll pick them up as you go. But in short, making levels is a simple, easy to learn process, but you'll need to learn how your tools work together (and I don't mean the coded combinations, I mean how item patterns work with other patterns)