Comment form – different title if no comment yet

Use get_comments_number() to get … well … the number of comments. In your theme’s comments.php you can use its return value to do something:

$num = (int) get_comments_number();

if ( 0 === $num )
    echo 'Be the first! But please don’t write just “First!”';
elseif ( 100 < $num )
    echo 'Yeah, add some noise. Nobody will read it.';
else
    echo 'Please write a comment';