--Christopher Hitchens (author "god is Not Great")
This post might get a little personal...

Matt Dillahunty of the Atheist Experience often uses an analogy to explain this distinction by comparing an atheistic view of god with a courtroom trial. Suppose a jury is trying to determine if a defendant is guilty or not guilty. Those in the "not guilty" camp include those who believe the defendant is innocent, and those who are simply not convinced of guilt. The same goes for atheism, which includes those who claim to know that no god exists and those who simply do not have an active belief in god (this also applies to theism, which includes those who firmly know of god's existence and those who simply believe). In a sense, atheists find god "not guilty" of existence.

First of all, I have yet to hear of a false belief which has absolutely no negative effects on a person's life, views or method of thinking. But I don't even have to go that far to show why such supposed "false yet positive" beliefs are actually harmful and why I am anti-theistic.
In one of the very first conversations I had with my mother about my atheism, she admitted to me that she has failed me as a mother. According to her, it was her job to prepare me as a youth to go out into the world and stay firm in my faith; to live in the world, but not of it. And since I no longer have faith in the religion of my upbringing, my mother did not adequately prepare me for adulthood; ergo, she has failed me.

The best I can hope for, according to Mormon theology, is the Terrestrial Kingdom (aka Heaven Jr) where my family can visit me on occasion (when they aren't busy building galaxies and making spirit babies). But if I, having gone through the temple endowment ceremony, am a son of perdition, even that consolation prize is taken off the table. This is what my mother fears most of all, and it is why she still prays for my speedy return to Mormonism.
This idea, although completely fatuous and rooted in a notion taken on faith rather than evidence, literally terrifies my mother to the point of tears. This is the damage mythology causes in the hearts and minds of true believers, however pleasant-sounding or otherwise inert it may seem. This is why I hate religion.
BONUS MATERIAL:
Some inspirational thoughts from Christopher Hitchens:
Matt Dillahunty explaining his courtroom analogy (part 1):
Part 2:
Part 3: