Extensions are an interesting new feature, since Firefox has been doing it for years, and Chrome has had them for much of its life so far. This is an area where we could start seeing the "walled garden" philosophy of the App Store taking hold. Safari extensions will be signed, and the certificate to sign an extension is only available through the Safari Developer Program. This means that extensions will probably have to be approved by Apple, and they may not be able to do anything with the file system. Extensions will be sandboxed like they are in other browsers.
Safari's Reader feature has technology from the "Readability" project of Arc90 Labs. It is enabled by an icon in the address bar that can take certain compatible articles and put them in a more readable format (without ads and other busy page attributes) that pops up in its own window. The feature has been included in the Instapaper program and in the form of Readability bookmarklets. The code is available under the Apache 2 licence so users of other browsers who want to make use of the Reader can create their own bookmarklet, with their own custom reformatted page style, font size, and margins.
Here are the other important new features in Safari 5:
- Hardware acceleration on Windows
- Bing can be your default search engine
- URL auto-complete is faster