Re: www?
There's no need for a special domain because there's already a special port.
I'm sorry John but that's rubbish, it would only apply if you had a single IP address with everything serving from a single host, but even back in the day that was considered unwise.
For DNS, you can't direct to a specific IP address using just the port.
At a minimum, a domain is going to have at least two Nameserver records: so ns0.domain.com and ns1.domain.com, which should be on separate IPs, and ideally separate subnets, then probably a mail server, e.g. mail.domain.com. Back in the day you would also often have an FTP server, ftp.domain.com and then a web server www.domain.com.
It is then very clear that if you want to talk to the mail server, you connect to mail.domain.com, if you want the ftp server you connect to ftp.domain.com, and if you want the website, you connect to www.domain.com, and the root nameservers know to connect to ns0 or ns1.domain.com.
That's why the convention was adopted, and the reasons for it haven't changed, in fact they are more relevant today than ever.
As mentioned below, if you want to use a CDN or DDOS protection or a loadbalancer or any other enhancement by use of CNAME records, you need to be able to distinguish the web host from the base domain and all the other sub-domains.