With commands, we can now easily create tools for the developers who work on our application. We know that commands have access to the whole service container. We also know how to make them rely as much as possible on services, making the code for the command available to the whole application, if we need it later.
We only saw one form of extensions for templates but know that all other extension types (with the exception of custom tags) are just as easy and straightforward to implement. Custom tags are quite complex, and they are also very rarely needed. You can learn the basics of creating a new tag at http://twig.sensiolabs.org/doc/advanced.html#tags.