Simple: It handles only the minimum necessary to implement HTTP/1.1. Well, maybe a little more than the minimum. Small: See the comparison chart. It also has a very small run-time size, since it does not fork and is very careful about memory allocation. Portable: It compiles cleanly on most any Unix-like OS, specifically including FreeBSD, SunOS 4, Solaris 2, BSD/OS, Linux, OSF. Fast: In typical use it's about as fast as the best full-featured servers (Apache, NCSA, Netscape). Under extreme load it's much faster. Secure: It goes to great lengths to protect the web server machine against attacks and breakins from other sites.
It also has one extremely useful feature (URL-traffic-based throttling) that no other server currently has. Plus, it supports IPv6 out of the box, no patching required.