When WP_DEBUG is set, WordPress sets (via wp_debug_mode() call early in core load process) the error reporting level to E_ALL & ~E_DEPRECATED & ~E_STRICT. This means all warnings and errors except strict errors and PHP deprecated functions (not WordPress ones).
You can define your own level in a custom mu-plugin (the override needs to be called as soon as possible, but after WordPress core load). Use this page for information about the error levels you can use. An example:
error_reporting(E_ERROR | E_WARNING | E_PARSE);