What are the major differences between Joomla! 2.5 and 3.x?
The most noticeable difference is the totally revamped administrator, which is updated to a modern design with many simpler and more friendly user-interfaces. Also, Joomla 3.0 is now mobile friendly and can be used with any modern device.
In addition, there are dozens of improvements to the details of all of Joomla, such as a new installer which you will notice when creating a new Joomla 3.0 install. Finally, Joomla 3.0 is packed with goodies for extension developers, such as Bootstrap support and jQuery support.