I agree it's not that difficult and prior to OpenSMTPD I've been a user of Postfix for 10 years, so I know the software is good and far easier to setup than the big S. ;-)

However, here's a better example of a configuration that is simple with OpenSMTPD and slightly more complex on others:

    listen on em0 tls cert "mycert" enable auth
    map "vmap" { source plain "/etc/mail/virtual" }
    accept from all for virtual "vmap" deliver to maildir
    accept for all relay

This will have the daemon listen on all addresses of interface em0 (both IPv4 and IPv6), it will enable STARTTLS using certificate "mycert" and activating authentication for system users (no pop-before-smtp, no cyrus-sasl and whatnot). It will accept mail from anywhere for all virtual domains in the mapping "vmap" and deliver to maildirs, while relaying mails from local users to the world.

That is a fairly basic setup that quite a lot of people use, yet the effort required to achieve similar setup on other software can range from just "slightly irritating" to "extremely painful". Here it's done with 4 lines that are almost readable by someone who has never used the software.

Some other features like relaying through remote MX that require auth; tagging; forcing secure channels; allow more complex setups while retaining the same simple syntax.

/!\ warning: as a major contributor to OpenSMTPD, I'm biased ;-) /!\