I've literally built hundreds of levels for Wizard. A couple of years ago, I found a box of 5 1/4" floppy disks labelled "JaSon'S LevELS!!!" (numbred 1 through 12) and decided to take a look. It was nice to see that one could notice a very clear progression in my understanding of both the tools and of basic design principals. Throughout the course of a single disk (which would hold about 50 levels) I could see how the levels became more and more intricate. And by disk 6, I started to see my "experimental" phase, using elements from the game for purposes other than their original intention.
Beginning at a very young age I was trying to learn and understand principals such as "What is fun?" and the idea of Risk Versus Reward. In my first levels, I would always litter the stage with the most valuable prizes (hey, I was ony 5), but I quickly learned that the player should have to work for the treasure.
Wizard was only the first level editor I've used, throughout the years I've enjoyed building DOOM maps with Waded and the DCK (Doom Construction Kit), Duke Nukem 3d build (probably my favourite level editor ever!), UnrealEd, and recently LittleBigPlanet. I'm a huge fan of level editors and believe that they are absolutely a great tool for anyone who wants to learn games and has any desire to make games.
That's why I (heart) Level Editors.