At comment preview the [...] are disturbing and BBcode is on my template frontpage off.
So thats how I solved it...
Stripping out with regex and 'non-greedy' matches we first extract the title and put it in a variable called $title_unformated, then we use the regex to extract the brackets including content and write it to the variable $title which will be displayed.
/components/com_comment/templates/default/readmore_preview.php from line 15 on:

<div class='ccomment-preview-container'>

	<?php foreach ($this->comments as $value) : ?>

		<?php

		if ($value->title != '') {

			$title_unformated = stripslashes($value->title);

			$title = preg_replace('/\[.*?\]/', '', $title_unformated);

		} else {

			$title_unformated = stripslashes($value->comment);

			$title = preg_replace('/\[.*?\]/', '', $title_unformated);

		}

		if (JString::strlen($title) > $previewLength) {

			$title = JString::substr($title, 0, $previewLength) . '...';

		}

		?>

There would be probably a quicker way, combining the stripslashes and preg_replace on one line, but I am a lazy guy and leave this to Daniel ;-)

Don't forget to do an override after testing, otherwise you will loose those changes with next update.
Override path would be: templates/MYTEMPLATE/html/com_comment/templates/default/readmore_preview.php

Hope this helps anyone!