WP: Add Bootstrap classes to comment form

comments.php

// Add to your theme's comments.php
<?php
	$comment_args = array(
		'class_submit' => 'btn btn-default submit',
		'comment_field' => '<p class="comment-form-comment"><label for="comment">' . _x( 'Comment', 'noun' ) . '</label> <textarea id="comment" name="comment" class="form-control" cols="45" rows="8" aria-required="true" required="required"></textarea></p>',
		'fields' => array(
			'author' => '<p class="comment-form-author">' . '<label for="author">' . __( 'Name' ) . ( $req ? ' <span class="required">*</span>' : '' ) . '</label> ' .
			'<input id="author" name="author" class="form-control" type="text" value="' . esc_attr( $commenter['comment_author'] ) . '" size="30"' . $aria_req . $html_req . ' /></p>',
			'email'  => '<p class="comment-form-email"><label for="email">' . __( 'Email' ) . ( $req ? ' <span class="required">*</span>' : '' ) . '</label> ' .
			'<input id="email" name="email" class="form-control" ' . ( $html5 ? 'type="email"' : 'type="text"' ) . ' value="' . esc_attr(  $commenter['comment_author_email'] ) . '" size="30" aria-describedby="email-notes"' . $aria_req . $html_req  . ' /></p>',
			'url'    => '<p class="comment-form-url"><label for="url">' . __( 'Website' ) . '</label> ' .
			'<input id="url" name="url" class="form-control" ' . ( $html5 ? 'type="url"' : 'type="text"' ) . ' value="' . esc_attr( $commenter['comment_author_url'] ) . '" size="30" /></p>',
			)
	);
	comment_form($comment_args);
?>

No problem. It was exactly what I needed. (Once I figured out the missing bit).