the previewbutton is shown only when you're logged in as an admin (ajax doesn't work there too).
edit: I just recognized, that my comment textarea fild with the name "comment" is invisible (display:none) and that there is another textarea field with name="comment-1b313a59" although that is not in the template. strange.
edit2: ok found the problem: it was the plugin antispam bee. it modified the name of the comment field. so it's not your plugin's fault.