Recently, causes I care about have been in the news. Sometimes for good reasons; the NFL now has its first active openly gay player (while numerous former players have been gay, none of them have been out while active; Michael Sam is out and has been drafted) and sometimes for bad reasons; the Supreme Court has expanded the scope of allowable ceremonial prayer, despite its previous acknowledgement that prayer endorsed by a government agency is unconstitutional. Both of these matter because of the concept of normativity. My friend Justin has written about heteronormativity over on Ninja Blues, with a particular focus on a controversy regarding a video game, but normativity extends beyond sexual orientation.
Bluntly, normativity is the assumption that everyone is exactly like you, except in the ways they differ from you. Everyone makes this assumption; I assume all the people I deal with daily are human, that they want to get things done, and that they're generally nice people. I also assume, with no rational basis, that they're probably heterosexual; that's heteronormativity at work. I try to avoid the assumptions with no basis, but I'm not perfect. Nor is anyone else, which is why the Supreme Court's decision matters so much.
There is a very strong assumption of Christianity in American culture. The language is soaked in religious references, and the default assumption of almost everyone is that any given person is a Christian. This leads to problems with trust; people distrust atheists, because they don't know who might be an atheist. We're invisible. That makes public prayer an issue, because now we must either fake a belief we simply don't have (and most atheists strongly dislike dishonesty) or expose ourselves and be vilified. That doesn't even touch the problems caused by religious people who aren't Christian being coerced into Christian prayer.
This is why atheists should be out of the closet. Silence in the face of encroachment is tacit permission. We need to speak up, to say "Actually, we do not believe as you do", and to be visible. Acceptance can only come when people know there is something to accept.