As John Lawler has explained, **"don't you do that"** adds emphasis to **"don't do that"**.  

That's what a mother would say to a child, a teacher to a naughty child, a wife to a reckless husband and, as Hot Licks points out, "dare" is implied most of the time.

>"William James, don't you do that! Put the cat down immediately."

Whether "don't do that" sounds more or less bossy, largely depends on the **intonation** one uses. It can be a friendly and loving **"don't do that...no, please..."** or a real order **"DON'T DO THAT !!!"**