I ran into the following issue with Simple Tags on WordPress trunk
Enabling Simple Tags on WordPress trunk causes queries of the form get_posts('tag=something') return all posts rather than just those tagged "something".
Though this problem was introduced by a recent change to WordPress, it seems like Simple Tags is causing the problem by overwriting the value of 'query_var' with '' in $GLOBALS['wp_taxonomies']. The following diff shows what $GLOBALS['wp_taxonomies'] looks like on a fresh WP trunk installation before and after enabling Simple Tags:
It seems like Simple Tags should at least ensure that these parameters are set to reasonable values, though I don't understand why it is necessary to call register_taxonomy() at all, since taxonomy.php already does this.
Thanks for your consideration,