In Symfony 3.4 you can disable global form themes for specific forms thanks to the new "only" keyword added to the "form_theme" Twig tag.
In Symfony 3.4, Guard authenticators can implement a new method called supports() to better separate the responsibilities of the getCredentials() method.
Symfony 3.4 adds a new command called debug:form to introspect all the information available for form types and their options.
Symfony 3.4 allows binding services and parameters in local configuration files to avoid duplicating them in multiple service definitions.
In Symfony 3.4 services and aliases are private by default. This may require you to change some service definitions in your bundles and applications.
Symfony 3.4 adds a minimalist PSR-3 logger so you can display, aggregate and store log messages without having to install Monolog.
In Symfony 3.4, there's a new directory to override third-party templates and a new feature to avoid circular exceptions when overriding and extending templates at the same time.
Symfony 3.4 provides a simpler way to inject all services tagged with a specific tag, so you don't have to create a compiler pass just to do that.
Symfony 3.4 adds new HTML5 form types ("tel" and "color"), new options for TimezoneType and support for callbacks in the delete_empty option.
In Symfony 3.4, the kernel of the application can implement CompilerPassInterface to define a compiler pass in it.