Please, see all the information for the end user here:
http://doc.tiki.org/tiki3
Also, this page: http://doc.tiki.org/Upgrade+2.x+to+3.x may be relevant.
Table of contents
Changes of behavior between 2 and 3
Structures are now optional
- You need to activate the structures feature if your site uses them (if you previously had structures and they "disappeared" after the upgrade to Tiki 3, you will need to turn the feature on again, on the wiki admin page). Naturally, the {toc} syntax for the structure table of contents will not work until structures are re-activated. Also be sure {toc} is enabled on the plugins admin page.
Strikethrough
New strikethrough wiki syntax can cause unintended surprises in your old documents. Ex: Removing strikethrough syntax
Security validation on many plugins
Many plugins like iframe, regex, etc, now require a trusted editor to validate
Themes
Theme Issues when Updating to Tiki 3.0
Some admin panels no longer accept html
Ex.: tiki-list_surveys.php
If you used html, you need to update text to use wiki syntax instead.
Known issues
Logo height limitations
Previously, the Strasa.css theme, which is used for the Featured Profiles, did not look good if a logo with a height greater than 100 pixels was used. But this bug was fixed in Tiki 3.1.
Some problems with installation in the root web directory on IIS has been reported, and are still being investigated
NOTE for devs: whenever this tw.o site is updated to latest tiki3 code, there could be a safe redirection, or just an IFRAME to avoid duplication of information (Xavi)