Also, there's no surprise that it's addictive, because you didn't include a natural place to stop. Every time you submit something, you're immediately given something new to do, no break inbetween! Of course people are going to have a hard time stopping.

Edit: Let me also add, it's not immediately obvious that there's a way to see what becomes of your chains if you don't keep going.

(Also there's too many people who will just totally ignore what they're supposed to do, but others have already said that and I have no idea what you can actually do about that.)

Edit: Also, the mismatch between the drawing box size and the display box size can be a problem if the boundaries of the picture matter.