Cameron,
The typical new Atheist argument is to argue that modern society agrees that certain principles are "moral". Under humanistic terms, what is becomes normative and so we find ourselves in a state where humanity has evolved socially to a point where certain things are "good" and others are "bad". On the basis of what society currently agrees is "right", they then judge ancient cultures as morally wrong. Do a search some time for the term "out of step" in News and you'll see it is a common term to refer to someone holding on to some outdated view of what is considered good. Reflexively, in our society, nobody wants to be viewed as "out of step" because it is considered bad because the prevailing view is that as man evolves so do societies and so does our sense of propriety.
The God of the Scirptures is "out of step" and all an atheist really needs to do to play to the unthinking masses is to appeal to the agreed upon sense that wherever the society currently rests on an issue is good.
This is obviously wrought with problems and suffers, logically, from the "is-ought" fallacy. This is what you're left with, however, when man is the measure of things. There is also a tremendous amount of philosophical naivete that assumes that the sense of morality that our culture has is an evolved sensibility ignoring the impact of wanton sin as well as the restraining effects that Christianity has had on Western culture.