The default configuration for many firewalls (software, at least) approximates "allow all outgoing connections but no incoming". Assuming all your connections p

The default configuration for many firewalls (software, at least) approximates "allow all outgoing connections but no incoming". Assuming all your connections pass through the same box (which they usually will, for a home network), there's no reason why it shouldn't have such a firewall even if it's otherwise a router that doesn't do NAT. Denying the ISPs trivial inspection into the structure of home networks, OTOH, is a major benefit and maybe a concrete downside to adopting IPv6 locally.