By Default, Turn Comments Off for Pages & Leave Comments On for Posts

Hook into wp_insert_post, check if it is an auto-draft for a page, and set the comment_status to closed:

add_action( 'wp_insert_post', 't5_disable_default_comments_on_pages', 10, 2 );

function t5_disable_default_comments_on_pages( $post_ID, $post )
{
    remove_filter( current_filter(), __FUNCTION__ );

    if ( 'auto-draft' !== $post->post_status or 'page' !== $post->post_type )
        return;

    $post->comment_status="closed";

    wp_update_post( $post );
}