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.

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

http://dev.tiki.org/wish2505

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)

Alias

• ReleaseNotes3.0