I often talk to clients about what constitutes "great logo design." I'll comment on things like balance, simplicity, positive vs. negative space and implied meaning. Most often these comments are met with logical recognition but I know, deep in my heart, they're not REEEEALLY understanding what I'm telling them. In these cases a picture is worth a thousand words.
Check out some truly beautiful graphic works by artist George Bokhua (via the Abduzeedo blog.)