You are correct, that you can learn most through experience, but that doesn't mean reviews aren't helpful too. Some reviews are short and pointless, but a long, well thought out review can be incredibly useful. It will give you a good look into the mind and opinions of the people playing your levels. (Or even other levels. You could check out the reviews of popular levels other people have made) You can get a good feel for what people think of the balance, power-up placement, etc. I think to truly understand gameplay theory, you're going to have to figure out what other people like and dislike. Because in the end, most of the people who you want to play your level are not you. And a level you created with balance and gimmicks that you really like could be something that other people just don't find fun.