1) Which hook is best used for field validation and display errors on the form? I tried to do it on BeforeCommentFormInsert() but the error is displayed on a different page. Is checking values in $_POST the correct way to check user input?

CommentFormSent()

2) This is totally unrelated, but I was testing out GetSpamKarmaForComment(), just put a single dummy line; return 100 in this function. Shouldn't the comment always be treated as spam when this is the case? But the comment was able to submit through and was saved into database. Is there a way to prevent this from happening if it's 100? Or this is the wrong hook to use?

You need to configure Antispam setting for delete threshold in order to use karma. Make sure it's < 100, e.g. 99

// Change status accordingly:
if( ! is_null($spam_karma) )
{
	if( $spam_karma < $Settings->get('antispam_threshold_publish') )
	{ // Publish:
		$this->set( 'status', 'published' );
	}
	elseif( $spam_karma > $Settings->get('antispam_threshold_delete') )
	{ // Delete/No insert:
		return false;
	}
}

1) I can't figure out how to validate the new field. A 'subject' field was added to the form. What is the value to check? Any examples?

2) Thanks, set the antispam threshold to 99 and it works!

Sorry, I mistyped the function name. I can now do validation in CommentFormSent(). What is the best way to pass information to display the error message on the form?

function DisplayCommentFormFieldset( & $params )
{
    // display a text input field
    $params['Form']->text_input( $this->get_class_id('myparam'), 'default value', 50, 'My param', 'Enter the value here' );
}

function CommentFormSent( & $params )
{
    $myparam = param( $this->get_class_id('myparam'), 'string' );

	if( empty($myparam) )
	{    // The field is empty, let's display an error and reject the comment
		$this->msg( 'You must enter the value', 'error' );
	}
}

Thanks for the example.