Ahh I think I know what you're talking about. Your game had an invisible wall which told me I couldn't cross it, but I didn't expect I could go over it until I accidentally discovered how. Many players won't do that and will get discouraged.
A little direction goes a long way, though I agree it's fun to figure things out based on intuition. I like how Valve handles tutorials in their games: they imply what you need to do without interrupting the gameplay.