A question for atheists

What is your primary reason or argument for asserting the nonexistence of God? Blackblade, for example, once mentioned the inferior design of the human body as being one reason for his doubts, which is a subject of some interest to me. I’m just curious about those that are outside of the eight general ones I addressed in TIA. And theists, do please keep your opinions to yourselves, as I’m no more interested in hearing your theories about why an atheist believes what he believes than I am in reading an atheist’s equally clueless theories about why you believe what you believe.

UPDATE: Correct me if I’m wrong, but in amidst the morass of tangential discussion, I have ascertained the following reasons:

Argument from Sub-optimal design (Blackblade)
Argument from insufficient evidence (Endosymbiosis, Eddie)
Argument from lack of scientific evidence (Garlic Powder)
Argument from failure to be impressed by religion (Jonathan B)
Argument from inability to believe (Noezoom)
Argument from unfamiliarity (Glasser)
Argument from material utility (Eddie)
Argument from post-trauma disorder (DH)
Argument from accident of circumstance (Stanford)
Argument from unconvincing arguments (Old Ned)

If I’ve missed any, other than the usual drivel from the oft-banned one, (how ironic that he, of all people, would complain of others being rude when he continues to impose himself upon us), do let me know. Or, if you feel I’ve failed to properly summarize your argument, I’d like to know that too.