It's based on KHTML, the rendering engine of KDE/Qt's Konqueror web browser, instead of on Gecko, the rendering engine of Mozilla. Don Melton explains why:
"The number one goal for developing Safari was to create the fastest web browser on Mac OS X. When we were evaluating technologies over a year ago, KHTML and KJS stood out. Not only were they the basis of an excellent modern and standards compliant web browser, they were also less than 140,000 lines of code. The size of your code and ease of development within that code made it a better choice for us than other open source projects. Your clean design was also a plus. And the small size of your code is a significant reason for our winning startup performance."
Translated through a de-weaselizer, this says:
"Even though some of us used to work on Mozilla, we have to admit that the Mozilla code is a gigantic, bloated mess, not to mention slow, and with an internal API so flamboyantly baroque that frankly we can't even comprehend where to begin. Also did we mention big and slow and incomprehensible?"
But I'm not bitter.
Update, Jan 14: Apparently the fact that Paul Festa linked here from his CNET article is going to reduce my Livejournal to the unadulterated depths of uselessness that the Slashdot forums have pioneered, so I guess I'll just turn off comments until the newbie shitstorm blows on by.
I'm not interested in your opinion. I'm not interested in explaining to you how you've completely missed the point of my post. I just don't care.
Thank you, drive through.
Update, May 7: (Shitstorm season over, presumably. It's a shame that turning off commenting also makes any existing comments be invisible.)