I've never been a single-issue voter. I think that those who base their decision solely on one issue are oversimplifying things. It is natural for atheists to want to know where each candidate stands on religion, but this is only one of the many factors that should be considered.
A vote for McCain is fundamentally a vote to maintain the status quo. Voters who are happy with the war in Iraq, believe the U.S. economy is doing fine, and are willing to continue giving up individual freedoms (e.g., privacy) for fear of terrorism should vote for McCain. Of the two candidates, he is most likely to take us to war with Iran. He is pinning everything on being a war president, and it may not be enough just to keep us in Iraq. In short, those who think that George W. Bush has been good for America and will be sorry to see him go should vote for McCain.
Obama presents himself as the change candidate. While he has been criticized for being more style than substance, it seems fairly clear that he offers much more of a departure from Bush. He wants to end the war in Iraq, and his economic plans are not simply about doing more of what led to the problems in the first place. I am not particularly convinced that he will go far enough in restoring the civil liberties we have lost under Bush, but he seems far more likely to work toward that goal than does McCain. Those who believe we need this sort of change should vote for Obama.
So yes, I think the decision will be a fairly simple one. Do we want a third Bush term or not? Are we happy with what America has become over the last 8 years, or would we welcome change?