Alter the comment form
You're overwriting the form elements that are already there, some of which are fields and so need to have their array structured very specifically (Why is hook_form_alter so messy in d7? has some background for that).

You can alter existing elements in the form without overwriting the whole thing like so:

$form['comment_body']['#prefix'] = '&lt;li class="comment_field_c"&gt;&lt;label for="comment"&gt;Comment&lt;/label&gt;';
$form['comment_body']['#suffix'] = '&lt;/li&gt;',