Allow Editors to edit pending posts but not draft ones

This is actually not hard. To add a new capability, call WP_Roles->add_cap(). You have to do this just once, because it will be stored in the database. So we use a plugin activation hook.

Note to other readers: All of the following code is plugin territory.

register_activation_hook( __FILE__, 'epp_add_cap' );

/**
 * Add new capability to "editor" role.
 *
 * @wp-hook "activate_" . __FILE__
 * @return  void
 */
function epp_add_cap()
{
    global $wp_roles;

    if ( ! isset( $wp_roles ) )
        $wp_roles = new WP_Roles;

    $wp_roles->add_cap( 'editor', 'edit_pending_posts' );
}

Now we have to filter all calls for …

current_user_can( $post_type_object->cap->edit_post, $post->ID );

… because that is how WordPress checks if an user can edit a post. Internally, this will be mapped to the edit_others_posts capability for other authors posts.

So we have to filter user_has_cap and look into our new edit_pending_posts capability when some wants to use the edit_post capability.

I have included delete_post too, because this also a kind of an edit.

Sound complicated, but it is really simple:

add_filter( 'user_has_cap', 'epp_filter_cap', 10, 3 );

/**
 * Allow editing others pending posts only with "edit_pending_posts" capability.
 * Administrators can still edit those posts.
 *
 * @wp-hook user_has_cap
 * @param   array $allcaps All the capabilities of the user
 * @param   array $caps    [0] Required capability ('edit_others_posts')
 * @param   array $args    [0] Requested capability
 *                         [1] User ID
 *                         [2] Post ID
 * @return  array
 */
function epp_filter_cap( $allcaps, $caps, $args )
{
    // Not our capability
    if ( ( 'edit_post' !== $args[0] && 'delete_post' !== $args[0] )
        or empty ( $allcaps['edit_pending_posts'] )
    )
        return $allcaps;

    $post = get_post( $args[2] );


    // Let users edit their own posts
    if ( (int) $args[1] === (int) $post->post_author
        and in_array(
            $post->post_status,
            array ( 'draft', 'pending', 'auto-draft' )
        )
    )
    {
        $allcaps[ $caps[0] ] = TRUE;
    }
    elseif ( 'pending' !== $post->post_status )
    { // Not our post status
        $allcaps[ $caps[0] ] = FALSE;
    }

    return $allcaps;
}