This game is great because the goal is so different from other video games. You aren't following a pre-designed path or figuring out the "right" way to beat the level. You have to come up with ideas of your own. That makes it hard to get at first--my son initially said "this is boring, it doesn't tell you what you have to do." But when I suggested he could give the guy almost anything he wanted--like maybe a flamethrower (just saying)--the lightbulb went on and the challenge shifted to how outlandish he could make his solutions. The limitation is the player's own imagination and vocabulary, and I think there's even been a couple of times when it helped teach my son how to spell a word correctly.
Now we actually have the most fun just playing around without a goal, seeing what crazy & funny things we can populate the empty levels with. And then blowing them up.