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.
- Content version control
- Many user interface improvements
- Easy multi-lingual setup for 64 officially supported languages
- Built-in Joomla! Extensions Finder as an onsite interface to the Joomla! Extensions Directory (that currently lists over 6,000 extensions) providing one-click extensions installation
- Increased security with strong passwords and two step authentication
- New rapid development framework for new extension coding
- Front-end Module Editing,
- Decoupling com_weblinks,
- Composer Integration,
- No CAPTCHA reCAPTCHA,
- Core en-GB standardisation...
- PHP 7 Support
- Update Notifications via Email
- System Information Export
- Drag & Drop Images
- Anonymised System Data
- Easily Insert Modules in Articles
- Item counter (+ click through filter) in Category Manager
- Random category blog and list order view.
- Show all menu items
- Menu type ACL
- Improved UX
- Categories on the fly
- Subform field
- Improved Joomla! Updates
- Allow stream connections to support proxies
- Build more complex database queries with new database query classes
- Delete all cache
- Delete and rebuild Extension Update Sites
- Easier template development with deeper JLayout integrations
- htaccess authentication for calls
- Improved Validation performance
- jQuery Update
- More Extension Details in System info
- Multilanguage ToS
- New “Showon” Option for fieldsets in component config
- New Log folder (for new installs only)
- New positioning settings for Bootstrap tooltips extended
- Protostar adds a new override for the Site Offline Page
- Reverse Caching Support
- SASS & SCSS File Support in Joomla! Template Manager
- Updated version of the CodeMirror editor.
- Custom Fields
- Multilingual Associations Component
- Article creation within the menu item
- Backend Menu Manager
- TinyMCE Improvements
- Share sessions between backend and frontend
- Allow packages to declare that their child extensions cannot be uninstalled
- List of all new features