Dave Hyatt comments in Surfin' Safari weblog about the changes:
What you are getting is all of the new standards support, new WebKit capabilites, site compatibility fixes and performance optimizations that are also present in Safari 2.0 for Tiger. The layout engines for the two are virtually identical.
Here are some of the highlights:
Page Load Performance
Safari 1.3 loads pages overall 35% faster than 1.2 as measured by IBench. In addition to improving the overall page load, Safari 1.3 will display content sooner than 1.2 did, so that subresources don't hold up the initial display of the page.
Did I mention it's really really fast? :)
One interesting point of note also is the claim by some that enabling Quartz 2D Extreme in Tiger 10.4 can slow down Safari 2.0's BenchJS performance significantly on some hardware.
Thanks to Screed and Squozen for pointing out that the Xmas calculation error is present in Safari 1.2 as well. There is no such problem with Mozilla Firefox in Windows 2000 or Microsoft Internet Explorer 5.2 in OS X 10.3.9.