Great thread. If I could add my own reasons -
Atheism for me was the result of a long journey. Along the way I came to realize that much of the bible was not supported by evidence that should have been there, such as the Exodus, the wars of the Israelites, and of course the utterly impossible and ridiculous global flood. Much of it is clearly rewrites of older stories that Christians today would consider pagan. Even many if not all of the supposed miracles of Jesus were copies of legends that were already common by the time Jesus was born, so much so that some biblical scholars, such as Bob Price, have said that Jesus did "nothing that was original to him". Eventually I came to accept that the bible is the work of man, full of all too human xenophobia, racism, sexism, class distinctions, and a blatantly wrong and ignorant view of the natural world.
So what is there to base belief in a god in without the bible? Would I have come to believe in a god if I hadn't grown up with religion? To put it another way, suppose for the sake of argument that you were born into a world where the man made bible, along with the other man made "holy" books, had never been written, but in which people had discovered all that we now know or have strong evidence for about the natural world: Common descent, the laws of nature, physics, the big bang, etc. Would you have any reason to postulate a magic all powerful invisible giant?
But religion is more than just wrong. It is detrimental to human development. There may have been a time when it served an important function, but that function has now been taken over by secular laws and science. Now it serves largely to keep people ignorant, to fight progress, and to make a virtue of self deception. Even worse, in many forms, it encourages and rewards murder, slavery, abuse of women, all the worst and most barbaric aspects of human nature. I do not believe religion in any form is worthy of respect. Religious individuals, yes. Some. But religion as an institution, absolutely not. I believe Sam Harris hit the nail on the head, that even religious moderates lay the groundwork and legitimize religious extremists.
Do I believe there is proof that god doesn't exist? Of course not. No one sane does. Granting that, if it makes you more comfortable, you may call me an agnostic. But I am agnostic to god only in the same respect that I am agnostic to fairies in my back yard that only materialize when no one is watching. Can I disprove that? No. Do I think it likely...