Makes the textarea the first field of the comment form.

t5-comment-textarea-on-top.php

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: T5 Comment Textarea On Top
 * Plugin URI:  http://toscho.de/?p=2239
 * Description: Makes the textarea the first field of the comment form.
 * Version:     2012.09.04
 * Author:      Thomas Scholz <info@toscho.de>
 * Author URI:  http://toscho.de
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

/* We use just one function for both jobs.
 * The first filter hooks in very late to let other plugins
 * (anti spam and such) do their job first.
 */
add_filter( 'comment_form_defaults', 't5_move_textarea', 100 );
add_action( 'comment_form_top',      't5_move_textarea' );

/**
 * Take the textarea code out of the default fields and print it on top.
 *
 * @wp-hook comment_form_defaults 100
 * @wp-hook comment_form_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;
	}

	// Now called on comment_form_top, $textarea is filled.
	print apply_filters( 'comment_form_field_comment', $textarea );
}

You should put this on the WP.org repository. If you update it, it'd be nice to get auto-updates.