An atomic blast is quite hot and spreads really fast. It will vaporize the body of a man anyway, and might even vaporize other stuff in the atmosphere and the surroundings. This is the one reason why atomic blasts are considered to be extremely devastating.
However, what is more devastating is the fact that the atomic blast leaves behind a big cloud of radioactivity which affects the surroundings for hundreds of years to come. This is the reason why many activists sign petitions to remove nuclear weapons from the world because they could mark the end of humankind.