Joomla! 3.3 / 3.4 required. Support for older Joomla! versions has been dropped. As a result overall performance has improved.
Feature cleanup. Several features which were only relevant in Joomla! 1.x / 2.x have been removed: Change Super User ID, Database Table Prefix Editor, Database Change Collation, Two Factor Authentication (we contributed a better version of this feature in Joomla! 3 itself!).
The Custom administrator directory feature was removed. This feature required some very specific server configuration to work. Unfortunately, most shared hosts didn't have it and couldn't enable it. Considering that this feature was offering little to no protection anyway we decided to remove it. You will get FAR BETTER enhancement of your site's security using the administrator secret URL parameter and administrator directory password protection of Admin Tools along with the Two Factor Authentication feature we have contributed to Joomla! itself. Enabling all three features and using different, strong passwords is adequate to deter all brute force login attacks far better than the custom administrator directory ever could.
Better IPv6 support. You can now define IPv6 CIDR ranger in black- and white-lists.
This version requires PHP 5.3.4 or later, due to show-stopper bugs in earlier versions of PHP 5.3. Important note on reading PHP versions: PHP 5.3.20 is newer than 5.3.10 which is newer than 5.3.4. Moreover, PHP 5.4.0 is newer than PHP 5.3.4. Joomla! 3.3 and later already requires PHP 5.3.10 or later. Also keep in mind that the entire PHP 5.3 series is end of life since August 2014 and MUST NOT be used on live sites any more for performance and security reasons.
This release requires any version in the Joomla! 3.3.x or 3.4.x family. It is NOT compatible with older versions of Joomla! including all 1.x, 2.x, 3.0.x, 3.1.x and 3.2.x releases. In any case, if your server doesn't meet the minimum requirements our pre-installation check asks Joomla! to cancel the installation and tell you exactly why it cannot be installed (minimum PHP or Joomla! version not satisfied).
Reading Joomla! versions: Joomla! 2.5.20 is newer than 2.5.10 which is newer than 2.5.6. Moreover, Joomla! 3.4.0 is newer than 3.3.6 which is newer than 2.5.28. In other words, you need to separately compare each one of the the three numbers separated by dots. These numbers are NOT decimals and trailing zeroes DO matter, i.e. 1.10 (pronounced "one dot ten", not one dot one oh) is larger than 1.2.