What specifically does he want to talk about that he thinks is now exempt from rational criticism?
Obviously, he doesn't think they're exempt from rational criticism, but he points out that many people do.
There are a lot of ways in which religions gets a free pass. One of the examples that Dawkins cites is the Supreme Court's ruling that Native American religious groups may not be prosecuted for using otherwise illegal hallucinogenic drugs as part of their religion, compared to the same court's ruling that people who use medical marijuana, even if it's legal in their state and used under a physician's advice, are subject to federal prosecution. "Imagine," he says, "members of an art appreciation society pleading in court that they 'believe' they need a hallucinogenic drug in order to enhance their understanding of Impressionist or Surrealist paintings."
He also cites the case of an Ohio boy who won in court the right to wear a t-shirt saying "Homosexuality is a sin, Islam is a lie, abortion is murder. Some issues are just black and white." School officials had told him not wear it to school. What Darwin finds interesting is that the case wasn't pursued as a free speech case, because they would likely have lost on free speech grounds. They claimed freedom of religion, though, and won.
He asks, "What's so special about religion that we grant it such uniquely privileged respect?"
I think the point is not so much what may be discussed, but the notion that in any discussion, religion trumps reason.