How to rearrange fields in comment_form()

That’s pretty simple. You just have to take the textarea out of the default fields – filter 'comment_form_defaults' – and print it on the action 'comment_form_top':

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: T5 Comment Textarea On Top
 * Description: Makes the textarea the first field of the comment form.
 * Version:     2012.04.30
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

// We use just one function for both jobs.
add_filter( 'comment_form_defaults', 't5_move_textarea' );
add_action( 'comment_form_top', 't5_move_textarea' );

/**
 * Take the textarea code out of the default fields and print it on top.
 *
 * @param  array $input Default fields if called as filter
 * @return string|void
 */
function t5_move_textarea( $input = array () )
{
    static $textarea="";

    if ( 'comment_form_defaults' === current_filter() )
    {
        // Copy the field to our internal variable …
        $textarea = $input['comment_field'];
        // … and remove it from the defaults array.
        $input['comment_field'] = '';
        return $input;
    }

    print apply_filters( 'comment_form_field_comment', $textarea );
}