Here is a list of changes that have been made between 5.1 and 5.2.

• Document Library Permissions - Model resources will not have VIEW by default (files, folders, shortcuts). See LEP-7277
• Journal Structure Inheritance - Structures will be able to inherit from a parent. Recursion is allowed. This will allow greater re-use, faster development and flexibility. See LEP-7263
• Improved Message Board to Email Integration - This allows for integration of Liferay MB with email inbox without use of a MX subdomain. Modified method signature in com.liferay.portal.kernel.pop.MessageListener from: public boolean accept(String from, String recipient); to public boolean accept(String from, String recipient, Message message); Will impact all EXT and Plugins environments that use pop message listener feature. See LEP-7283
• Refactored Message Bus Configuration - Separate configuration details for Liferay Portal's internal message bus from code. Refactoring allows for configuration of message bus via spring configuration file. See LEP-7303
• Moved DataHandler interface to a different package: all classes within com.liferay.portal.kernel.lar have been moved to com.liferay.portal.lar in portal-service.jar to provide more facilities to DataHandler implementors.
• API Change: This change will require changing the imports in the existing DataHandlers. See LEP-7595 for details
• Portal administration centralized in a new Control Panel: All registered users will have access to it although will only see the different admin options according to their permissions. The old individual admin portlets are still available but cannot be added to new pages (if you want to keep the old behavior and allow users to add them you can achieve this by changing a configuration option. Check LEP-7550 for details)
• Support for custom types of organizations: it is now possible to define new types of organizations (previously there were two fixed types: regular and location).
• API Change: The methods addOrganization() and updateOrganization() of OrganizationService receive the type of organization as an String instead of as a int. Also the interface Organization has changed the field type from int to String. Finally the following methods have been removed:
• Organization: getTypeLabel(), isParentable(), isLocation(), isRegular()
• User: getLocation(), getLocationId(), hasLocation()
• Configurable Terms Of Use: see How to Create a Dynamic Page for Terms of Use
• Import Wordpress Blog into Liferay Blog: see How to Import Wordpress Blogs into Liferay Blogs
• Custom User Attributes: see How to use Custom Attributes to enhance Users
• The login portlet and the open-id login portlet have been merged so that there is no longer a need to add both separately. If open id is disabled the open id fields will not be shown.
• If you were using the open-id portlet it will automatically disappear

More to come