The most immediately noticeable new feature is Safari Reader, which generates a simplified view of any Web page that contains a primary article. The simplified view is black text on a white background and slides up over the originating Web page -- you call it up by clicking the "Reader" button in the address bar.
The new sheet also displays icons for zooming in and out and printing or e-mailing the simplified view. The e-mailing feature didn't work for this writer, though -- it popped up a message saying it couldn't find an e-mail program, even though my default reader is Apple's own Mail.
It's a very nice feature to have built-in, though the third-party add-on Readability already does much the same thing and offers more customization options.
Safari 5 also implements DNS prefetching, by which it looks up the addresses for all the links on a Web page before you click on them, which eliminates that small delay when you click on one. Apple also claims to have improved page caching for faster loading of already visited pages.
For more new features, see Page 2.
The new version of the browser also supports "over a dozen" new HTML5 features, such as geolocation and full-screen video. Apple recently put up an HTML5 Showcase page to show off some of what HTML5 can do -- unfortunately, it only works in Safari, not because other browsers don't support the commands, necessarily, but because Apple blocks them from even trying. Bad Apple.
One of the more intriguing new features is the introduction of Safari Extensions, which enables third-party developers to "enhance and customize the browsing experience." The developer program is free to join and gives members sample code, testing and debugging tools, and more.
A Safari Extensions Gallery is set to open later this summer to let consumers browse and download extensions.
The ability to add features via extensions and plugins has long been one of the reasons Firefox users prefer their browser to Safari. It remains to be seen, of course, if Safari can match the extensive Firefox ecosystem.
Other new features include a Smart Address Field, which supposedly searches the titles pages in your history and bookmarks to try and match anything you type into the address bar; hardware acceleration support for Windows PCs; and a Bing option in the Search field.
For more details about the new Safari and its system requirements, see "New 'Nitro-powered' Safari 5 is the fastest, says Apple."