New to this version of Safari is the ability to save webpages as "Web Applications." This new feature allows Safari to save pages similar to the way Fluid does. You also have the ability to choose how new windows will open (i.e. with your favorite bookmark, blank page, etc.).
One of the biggest "features" is the fact that Safari 4 (along with the new version of WebKit) scored a perfect 100/100 on the Acid3 test. If you are inclined to see what the new version looks like, World of Apple has placed some screen grabs on their blog for your viewing pleasure.