1. change php_short_tag
<?=)to full tag
<?php echo, as in some php environments, the default php_short_tag support is set to off
2. "Categories to Exclude" is confuse for me, I think it mean that posts in the selected categories will not be included in related posts, but is real mean that if the current post is in the selected categories the plugin will ignore and skip export related posts contents.
3. I think it will be better if you can provide a options to which categories will be count when find related posts
4. when there is no related posts found, the plugin will return nothing, even ignore the before and after widget content, this behavior will break the document structure sometimes. It will be better if return a empty content with before and after widget content structure.