Quirks mode and DOCTYPE

By now every web developer worth their spit knows that the lack of a valid DOCTYPE will trigger "quirks mode" in modern browsers (IE 6 on Windows, IE 5 on Mac OS X, and Mozilla on any platform). Quirks mode is a backward compatibility feature to simulate rendering bugs in previous versions of the browsers (IE 5's box-model problem, Netscape 4's everything).

But exactly what are the quirks that quirks mode triggers? WebDesign-L pointed me to Craig Saila's DOCTYPE reference, which pointed me to these resources:

1. Quirks mode in IE 6 on Windows
2. Quirks mode in IE 5 on Mac OS X
3. Quirks mode in Mozilla