The new "autoconfigure" option adds automation to the "_instanceof" option, greatly simplifying the configuration of your services in Symfony 3.3 applications.
In Symfony 3.3, the deprecation messages generated during the compilation of the service container will be persisted, so they won't disappear when you reload the page.
Symfony 3.3 will include a new component called WebLink to manage links between resources; implementing features such as preloading and prefetching.
In Symfony 3.3, the SecuritBundle includes improvements such as a smarter security:encode-password command and a more robust logout URL generation.
Symfony 3.3 has redesigned the exception pages to modernize them and match their design with the toolbar and the profiler.
In Symfony 3.3 we optimized the performance of the routing component to make your applications faster, specially when they define a lot of routes.
Symfony 3.3 defines a new Kernel::getProjectDir() to replace Kernel::getRootDir() as the way to find the root project directory.
In Symfony 3.3, it's easier to work with flash messages in templates. In addition, the session no longer starts automatically when checking if there are flash messages.
In Symfony 3.3, the Workflow component will include lots of useful new features.
In Symfony 3.3, the special SYMFONY__ environment variables have been deprecated and they will no longer work on Symfony 4.0. Use regular environment variables instead.