Why isn't the code in my implementation of hook_form_alter() being called?
First of all - have you checked that Drupal sees your function at all, and calls it?

Then, have you tried printing out $form_id? As for default D7's content types (article and basic page) comment form ids are not "comment_form", but:


comment_node_article_form - for article
comment_node_page_form - for basic page