• The framework is released under the MIT license
  
• The database drivers have had extensive refactoring
  
• PDO is fully functional with subdrivers
  
• There is a new Session library
  
• There is a new Encryption library
  
• The unit testing has been beefed up, and code coverage improved
  
• PHP 5.4 or newer is recommended, but CI will still work on PHP 5.2.4

http://forum.codeigniter.com/thread-1657-post-7480.html#pid7480