What are the major differences between Joomla! 2.5 and 3.x?
From Joomla! Documentation
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, starting with Joomla! 3.0, Joomla! is now device responsive. Simply, Joomla! is mobile friendly and can be used with any modern device.
In addition, there are dozens of improvements to the details of all of the Joomla core. Some highlights are:
A new installer which you will notice when creating a new Joomla! 3.x install.- Joomla! 3 is packed with goodies for extension developers, such as Bootstrap support and jQuery support.
- Joomla! 3.1 adds Tagging capability.
- Joomla! 3.4
- Front-end Module Editing,
- Decoupling com_weblinks,
- Composer Integration,
- No CAPTCHA reCAPTCHA,
- Core en-GB standardisation...