One of the things they mostly do is try present themselves as more moral than God. Usually they focus on the Old Testament and try to turn God into a monster "God killed people, promoted genocide, is this really a loving God, blah, blah, blah..."
First off, that is a really weak argument against the existence of God. As pretty much their argument is that: "I don't agree with God's morals and so God is a monster and this not my picture of what God should be - so therefore God does not exist."
So it's like some one saying:
"I don't agree with Obama's policies - so therefore Obama doesn't exist."
Just because you don't agree with someone or something - doesn't mean that they don't exist. Obama exists (unfortunately

Another thing they point out and gripe about is some of the actions of people in the Bible - IE Lot's incestuous relationship with his daughters. But obviously The Bible is simply recording what happened - it does not mean that God approved of every person's actions. God did not obviously approve of David's adultery - but it is still in the Bible because that is what David did. It is like writing a book on facts of the assassination of Abraham Lincoln and then someone saying that we support assassinating presidents - no we don't - we are just recording the facts of what happened in history. So just because they are gross sins mentioned in the Bible does not mean that God is some kind of sicko - unfortunately some people are sickos - and God felt to record their actions in His Word. Usually the sins recorded in the Bible serve as a warning to us - these people sinned and look what happened to them - don't do what they did - or the same thing could happen to you.