Please, see all the information for the end user here:
http://doc.tikiwiki.org/tiki3

Also, this page: http://doc.tikiwiki.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 if you really use them

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 Tiki3.0

Known issues

Logo height limitations

The Strasa.css theme that is used for the Featured Profiles will not look good if a logo with a height greater than 100 pixels is used.

If a logo greater than 100 pixels in height is used, in IE7/8, Firefox, Opera, and Safari, the logo pushes the login bar farther down the page, making a white gap between the dark blue and bright blue areas; and in IE6, there's no white space below the dark blue header top area and above the bright blue login area, but the logo extends under the login form (logo covered by form), and is visible below the form.

Some problems with installation in the root web directory on IIS has been reported, and are still being investigated

http://dev.tikiwiki.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)