* Enhancements of System Configuration:
  * Introduced new items that can be configured in the similar to Magento 1.x way, using xml files: nested groups in System Configuration form, group dependencies, intersected dependencies
  * Enhanced handling of field dependencies, required fields functionality
  * Changed Configuration structure to be represented as an object model
  * Improved performance of configuration rendering
* Implemented new API in `Mage_Webapi` module
  * Removed `Mage_Api` and `Mage_Api2` modules as obsolete API implementation
  * Added support of REST and SOAP 1.2 [WS-I 2.0](http://ws-i.org/Profiles/BasicProfile-2.0-2010-11-09.html) APIs
  * Introduced versioning per API resource. The application will support old version(s) of API after upgrading to not make old API requests fail
  * Unified implementation for all API types
  * Significantly simplified coverage of new API resources
  * Added two-legged `OAuth` 1.0 for REST authentication
  * Added WS-Security for SOAP authentication
  * Added automatic generation of REST routes and SOAP WSDL on the basis of API class interface and annotations
  * Introduced generation of API reference from annotated WSDL (for SOAP API)
* Introduced service layer. Business logic should be implemented once on service layer and could be utilized from different types of controller (e.g., general or API)
  * Business logic is implemented on service layer to be utilized from different types of controller (e.g., general or API)
  * Implemented abstract service layer class - `Mage_Core_Service_ServiceAbstract`
  * Implemented concrete service layers for customers, orders and quotes. Appropriate duplicate logic has been eliminated from controllers and API
* Improved validation approach:
  * Added support of describing validation rules in a module's configuration file - `validation.xml` in the module's `etc` directory
  * Added `Mage_Core_Model_Validator_Factory`
  * Added new validators to Magento Validator library
  * Added `Magento_Translate_Adapter` as a translator for the validators
  * New approach is utilized in `Mage_Customer`, `Mage_Eav` and `Mage_Webapi` modules
* Added profiling of DB and cache requests
* Minor Improvements:
  * Added an ability to choose the image for logo and upload it from backend web-interface
  * Added notification in backend in case of product SKU change
* Bug fixes:
  * Fixed bug in `Mage_Adminhtml_Sales_Order_CreditmemoController` that changed item’s stock status after each comment
  * Removed `Debug` section in `System -> Configuration -> Advanced -> Developer` for default configuration scope
  * Fixed bug in `Mage_Tax_Model_Resource_Calculation` that prevented placing order with two tax rules having the same rate
  * Removed `Url Options` section in `System -> Configuration -> General -> Web` for website and store configuration scope
  * Changed backend template for UPS shipping provider to fix translation issue
* Fixed security issue - set `CURLOPT_SSL_VERIFYPEER` to `true` by default in cUrl calls
* Added `Zend/Escaper`, `Zend/I18`, `Zend/Validator` ZF2 libraries
* Updated `Zend/Server` and `Zend/Soap` libraries to ZF2 versions