Here is a brief list of the changes from Changes Made Between Preview 3 and Beta (these and much more are contained within the release notes also available on the download page):
- New Simple Membership Features in the Default Project Template
- New Filter Types for Authorization and Exception Handling
- New Output Cache Filter
- Namespaces in Routes
- New Interface for Enhanced Testability of TempData
- ActionInvoker Extensibility Improvements
Scott Hanselman has a great blog post about what's new with the release. You can view it on Computer Zen.