Here is a list of some of the changes that has been intriduced into WordPress 2.3.
1. Native tagging support – in addition to categories, tags are now natively supported by WordPress. Importers for Ultimate Tag Warrior, Jerome’s Keywords, Simple Tags, and Bunny’s Technorati Tag plugins are included.
Update Notifications – if an update for WordPress or a plugin is available, a message is displayed indicating.
2. Canonical URLs – enforces no-www preference and attempts to redirect incomplete URLs or URLs to posts with changed slugs to the correct location.
3. Pending Review – now a contributing writer can submit a post for review by an administrator or editor.
4. More features in WYSIWYG Editing – additional features of TinyMCE have been exposed.
5. Full Atom 1.0 Support – including the publishing protocol.
7. New taxonomy system – post and link categories have been combined with tags.
8. Ability to disable update notifications – in case you don’t want these notifications.
9. Pluggable Importers – you should be able to write or use plugins to import from additional sources.
10. 351 Bug fixes – which is nice