From the release-notes.html:
What's New?
New User-Interface Color/Style
Higher-Level Categorization of Bugs (above "Product")
Regular Reports by Email of Complex Queries ("Whining")
"Environment Variable" Authentication Method
User-List Drop-Down Menus
Server-Side Comment Wrapping
UI for Editing Priority, OS, Platform, and Severity
Bugzilla Queries as RSS
Choice of E-Mail Sending Methods
"Large Attachment" Storage
and lots of Miscellaneous Improvements
See http://www.bugzilla.org/releases/2.20/release-notes.html for
all the details.
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