hypothesis: the Great Filter is that every sufficiently long-lived society eventually invents and uses the Deplorable Word from C.S. Lewis's "Narnia" universe.
hypothesis: the great filter is simply an adequate understanding of physics. The galaxy (and beyond?) is happily chattering away using physics we do not understand. We are a pre-contact tribe in the jungle completely unaware of the "radio" signals passing all around us. Once we understand the principles, we can build such a radio and participate.