Fast (917tps vs WP's 287tps).
Flexible. Dropped the PHP 4 architecture that it shared with WP. Most systems are abstracted away. Uses a service container with dependency injection. Can be built using Composer. All configuration lives in code.
Easy to develop on. Uses modern PHP best practices and principles. Took this non-front-end guy less than six hours to convert a WrapBootstrap theme to Drupal while reading the docs for the first time.