In addition to many bug fixes and performance improvements, some specifics include:
New Features & Settings
Legacy PayPal gateway to allow for older PayPal Subscriptions to still be recognised.
Attachment Type Restrictions: You can now choose from three options for file attachments in any post content: No files, Images only (this will actually check to make sure an uploaded file is a valid image) or All files (this is default) - If you choose "All files" you have the option to provide a list of allowed file extensions.
Merge Concurrent Posts: If a user makes a post or comment and then makes another post or comment on the same item before any other user, the posts will be merged. There is a setting to disable or set the time limit for this to happen. There is a clear, visual indication when a merge happens to avoid confusion to the end user.
Moderators can click and hold on topic and other item titles to quickly edit.
New setting to optionally set products in Commerce to have a grace period to remain active for a period of time after expiration.
New setting to specify domains to exclude from having rel='nofollow' added to links.
The "+ Create" menu at the top of the page now has an option to create an announcement.
New button to post a status within the status updates widget
You can now embed Pages database entries by pasting their link.
New widgets for blog entry, gallery images and downloads files feeds.
Widgets displaying content feeds can now be given a custom title.
The subject lines for emails sent about new content have been changed slightly so email clients can group them better.
Some widgets providing duplicate functionality have been merged. For example, "Latest Topics" has been merged into "Topic Feed".
Announcements now show on every page by default (but can still be removed).
When upgrading, if you have a large database you are presented with an option to run queries manually. You can now opt to run them anyway.
New ability to disable but not uninstall an app.
IPS apps that are not up to date will be disabled on upgrade to prevent things breaking.
The Support system in the AdminCP will now disable non-IPS apps in addition to all plugins as part of its diagnostics. It will also show you a list if what it is disabling to help you find problems.
Notes for translators
The public-facing navigation tabs now have language strings separate from the application title (this means translators will need to re-translate the application tabs upon upgrade).
188.8.131.52 was released to fix:
Making a post might not work on older versions of PHP
Editing permissions for a module may not work
Issues with widgets after upgrading
Some clients seeing apps turning offline on upgrade
184.108.40.206 was released to fix:
Fixes an issue where comments on Blog entries may not work
Fixes an issue where custom drop down fields in Pages databases may not work
Security and privacy enhancement for those using file system data storage option
Security and privacy enhancement for system-generated log files
Critical bug fixes for common problems reported from clients