Nothing, it's all unbelief.
What's the difference between black, and jet black? A few shades of black?
Same thing more than likely, a few more things they do not believe in, or more things that that have an unbelief about.
What I find funny, is that atheist are creating their own seperate atheist groups, their own little churches if I may use a biblical term, which means no more or less, than a group of people gathered together for a purpose. Yep, thats what it means, look it up in the greek.
Wherever you have a group of people gathered for a specific purpose its a church according to the biblical greek definition. So, all of you here are a part of the TwoPlusTwo God, Bible and Christian bashing Church.