In no particular order:
1) God sacrificed himself to himself for the curse that he placed upon humanity. This just seems like a really retarded god.
2) The book doesn't give me that important, "Hey, this must have been written by highly inspired people that consulted with some very powerful being" feeling.
3) God's a spoiled, evil prick.
4) God kills infants/children to suit his own purposes.
5) God's failed creation. Adam and Eve were failures . The entire world failed, so god destroyed everything but Noah. The Israelites get killed off by neighboring nations and god himself until only a chosen few make it to the promised land. Warmonger anyone?
6) No geological evidence for the flood.
7) No substantial archaeological evidence for the Bible.
8) Christians are such hypocrites for the most part.
9) No reason to believe that the Bible is more believable than any other religious text.
10) No way to objectively tell what the Bible means. Fundamentalists contend that everything is literally true except for the most obviously metaphorical parts, while liberals state that most of it is just allegorical.
11) No contemporaneous sources for New Testament stories.
12) Natural disasters.
13) Science makes more sense.
14) People are gullible, think Scientology.
15) Stoning as a method of killing.
16) God doesn't try to communicate to us through the 5 senses he supposedly instilled in us.
17) Lack of evidence.
18) Christians think the must stupid, mundane things are evidence of god and his involvement in human affairs. For example, a tree by Caylee Anthony's memorial site was struck by lightning, so that implies that god was involved. Where was he when she died?
19) Statistically speaking, Christians fair no better in married relationships than non-Christians.
20) Poor education levels, unwanted pregnancies, and violence are more highly coorelated with Christians.
*Bonus* What Harris said.