What are the major differences between Joomla! 2.5 and 3.x?

From Joomla! ドキュメンテーション

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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:

Star icon.png A new installer which you will notice when creating a new Joomla 3.x install.
Star icon.png Joomla 3 is packed with goodies for extension developers, such as Bootstrap support and jQuery support.
Star icon.png Joomla 3.1 adds Tagging capability.