Most importantly, this Beta sees the integration of Adobe Flash into the browser itself. Not only should this be faster, it should also be more secure -- and stable, because patches for Flash will now be performed by Chrome's built-in auto-updater! These won't be the slow-release once-a-year- Flash releases either -- Google Chrome has a direct line to the Adobe developers to make sure patches happen quickly.
All in all, this is a monumental update -- I urge you to give the Chrome 5 Beta a go. I'm a massive Firefox fan, but it sure feels like Chrome is accelerating away from the pack.
[As an aside, there's a fun video after the break about how the Google team behind V8 designed and implemented speed tests to measure just how good V8 really is]