Correction: If the smallest and largest are set to the same size, WP will still make the tags different sizes. I think this is a bug. Work-around is to make the two sizes just a little different, but not enough to be noticeable.
<?php
$tag_list = wp_tag_cloud('format=array&orderby=name&order=ASC&smallest=1&largest=1.001&unit=em&number=0');
if (!empty($tag_list)) {
echo "<p>Tags:</p><ul>";
foreach ($tag_list as $key => $tag) {
echo "<li>$tag</li>";
}
echo "</ul></div><!-- end tags -->";
}
?>
There should be a WP function to provide a simple non-cloud-style list.