The extensions framework for Google Chrome has been one of its most anticipated features. Many people rely on several extensions in Firefox, so the arrival of extensions in Chrome should certainly compel a new group of users to switch to Google's browser. The gallery is already packed with them - including several ad blockers. My favourite extension is Flashblock, which blocks all Flash content by default, making it accessible through a single click.
Another new feature in Chrome 4.0 is bookmark sync. This handy little feature will allow you to synchronise your bookmarks (really?) across different machines, so you don't have to manually recreate/manage them for each machine. You'll need a Google account for it, though.
Web developers can rejoice too with this new release, as it brings support for several HTML5 features, like LocalStorage, Database API, WebSockets, and more. Another newcomer for developers is the notification API (Windows-only, for now), which allows web pages to send notifications to the browser in a non-annoying way. This feature is obviously important for ChromeOS.
Sadly, Chrome 4.0 is only stable on Windows. Linux users are on the beta channel at the moment, but are more or less at feature parity with the stable Windows version (except for bookmark sync), and in all honesty - I don't have a single stability problem with the Linux beta channel. Mac users still have a lot to wait for though, as they don't even have extensions yet.