New Features and Improvements:
- Experimental SQLite Support
- Creating an Attachment by Pasting Text Into a Text Field
- HTML Bugmail (default: on  can be disabled in user preference)
- Improved Searching System
- Disabling Old Components, Versions and Milestones
- Displaying a Custom Field Value Based on Multiple Values of Another Field
- Auditing of All Changes Within Bugzilla
- Accessibility Improvements

And many other Improvements, for complete list see:
 http://www.bugzilla.org/releases/4.2.1/release-notes.html