Atheism is, I think, the decisive rejection of the idea that the supernatural is a good way to explain the world. Might there be some sort of supernatural being? I dunno. More importantly, what does the phrase "supernatural being" even mean? If we look at the ideas we've traditionally ascribed to the word "god", we consistently see ideas of one or more personal, teleological beings whose will and desire are explanations for observed phenomena. The pagan gods made the lightning strike and made winter occur; the Christian god performs wonders and miracles. The ideas of god historically underlie political legitimacy: The king and the priesthood rule by direct dispensation from the local deity. The ideas of god historically provide psychological comfort: In the gods, one can rely on a responsible parent, who will at least provide sustenance and justice. All of these ideas are wrong.
To reject these ideas is more than just to eliminate the statement, "I believe some God exists" from one's mind. Yes, that's the dictionary definition of atheism, and it has some use against various theistic straw men, but anything more than the most trivial, nominal atheism requires rejection of the supernatural. To the atheist, science becomes the way to explain the world (or, more precisely, the way to construct explanations). Only our own moral intuitions — as subjective, limited and labile as they might be — underlie political legitimacy. The atheist relies on himself or herself for psychological comfort. We are, in short, free beings in a natural world, free to shape that world how we will, subject only to the constraints imposed by the impersonal and uncaring laws of physics. We are as well free to fail; there is no parent who will swoop in and rescue us from our mistakes or catastrophes. To me — because I am a mature adult — such freedom is an unqualified good.
If you're religious, I'm going to ask you two questions: What fear are you unable to face? Whom do you want to oppress? If I don't get an answer to either question, I'm going to press you on how you consider yourself religious in the first place. Vague mystical bullshit doesn't count: A difference that makes no difference is no difference, and there's no difference between a stone-cold atheist like myself and a theist who relies exclusively on mystical mumbo-jumbo. (If you want to pretend you're religious to get along in society, that's your own business. But do us all a favor and don't be a hypocritical jackass; refrain from attacking your fellow atheists to get on the good side of the religious. It doesn't work anyway; they'll hate you more for lying.) But 99 times out of 100, underneath the mystical mumbo-jumbo, I'll find an answer to one of those questions.
Eight times out of a ten it'll be the last question, and the answer will be niggers, fags, sluts and/or the lazy, slovenly, dirty poor*. There will always be some elaborate rationalization, of course — few people can stomach being openly racist, homophobic, sexist, or classist — but I'm completely uninterested in how you rationalize being an asshole. Yes, 80% of religious people are just garden-variety bigots. (Most of rest are usually terrified of disappointing their (real) mommy and daddy. I don't buy the whole "fear of death" bullshit; anyone who gets in a car or crosses the street is not really afraid of death. The few remaining are just there for the show, which I will admit can be quite entertaining, if that's your cup of tea.)
The "philosophy" of theism is tiresome. The whole edifice of philosophical bullshit around theism exists only rationalize and obfuscate assholery and cowardice. I'm at the point where if you call yourself a Christian or a Muslim I'm just going to silently conclude you're a bigot and/or a coward. I might be polite to your face (unless you say something openly bigoted or ridiculous), but I'll always have a reserve of contempt for your character, intelligence and maturity.
*I can usually do it in four questions: Do you oppose affirmative action? Do you oppose gay marriage? Do you oppose abortion on demand? Are you a Republican? If you answer any of these questions in the affirmative, please remember that I'm not in the least bit interested in how you rationalize being an asshole.
It's true, of course, that one can find other reasons to a bigot or a coward; bigotry and cowardice might be the only reasons to be religious, but religion is hardly the only way to rationalize bigotry and cowardice. I'm no more approve of non-religious justifications for assholery (e.g. Libertarianism or Randianism) than I approve of theism, but I'm only one guy, and I have only so much time and energy.
Fundamentally, that's what atheism really is: science, personal moral responsibility (if you're going to be an asshole, at least be one on your own account, not a god's), and adult psychological maturity.