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.