The first, obviously, is instructions on how to construct a reasonable packet-filtering firewall. And somewhere in there, I hope people get the idea that every single service and program they run on an Internet-connected machine has its own security implications. Some services are not designed for use over the Internet. And some of today's popular services cannot be protected at the packet-filtering level. The protocols require some kind of more intelligent proxy.
Security is such a broad topic.
I'm a geek who's into packet-filtering, which is a pretty small piece of the pie. Linux is preparing to support IPv6, which supports authentication, encryption, and digital signatures at the IP level. IP-level support for authentication will potentially make it far more difficult to spoof source addresses in packet headers. IP source address spoofing is a major tool in denial of service attacks. Encryption support and an incorporated IPsec implementation can provide for transparent VPN tunneling.