Show a very long list of details about your machine and kernel (verbose output)
The entire list of system properties using -a for "all properties" (verbose output). Just looking through the output you'll learn something new about your system.
Last time the system was booted (1 line output)
Swap usage (1 line output)
How many keys does sysctl have values for? 644.
$ sysctl -a | sed '/^$/d' | wc -l # not counting blank lines 644
What top level categories does sysctl have values for?
$ sysctl -a | sed '/^$/d' | sort | cut -d. -f1 | uniq appleprofile debug hw kern machdep net security user vfs vm
Operating system version (3 lines output)
Total current uptime (1 line output)