According to the Bugzilla 3.0 release announcement, some of the new features in this version include custom fields, support for the Apache mod_perl module, per-product permissions, an XML-RPC interface, and the ability to create and edit bugs via email. A demo site has been set up where users can test the new version before downloading.
In his blog, Dave Miller, the current Bugzilla maintainer, explains why it took nine years for Bugzilla to move from 2.0 to 3.0. He mentions that several years ago Ian Hickson, a Bugzilla developer, attempted to create Bugzilla 3.0 by rewriting the code from scratch. Because of that, new Bugzilla versions were released in the 2.x branch. Eventually, though, "the vast majority of the design goals from Hixie's Bugzilla 3 proposal got met along the way, by iterative development of the existing codebase." Since the original idea was obsolete, the Bugzilla developers prepared a roadmap for 3.0 last year which led to this release.
Along with several new features, the new version boasts a streamlined and stable code base, due to its long release cycle and extensive testing. Bugzilla was initially written in the TCL programming language, before being ported to Perl in version 2.0. While the latest release is still in Perl, Max Kanat-Alexander, Bugzilla's release manager, in a recent blog post argues that Perl isn't the right choice anymore, citing advantages and disadvantages of several other languages.
When I last checked, Mozilla's Bugzilla itself hadn't been upgraded to 3.0; the project is still using the older 2.23.4 version. Meanwhile, development on the next stable release, Bugzilla 3.2, is already underway.