I would not take visits into account: To get these numbers you have to connect to an external table, or worse write into the WordPress tables on each request.
Use comments, that is built-in. Here is a sample code that adjusts itself to your comment development:
add_filter( 'the_content', 'wpse_78513_hotness' );
function wpse_78513_hotness( $content )
static $average = FALSE;
static $global_stats = NULL;
if ( FALSE === $average )
$average = round( wp_count_posts()->publish / wp_count_comments()->approved );
$current_comments = wp_count_comments( get_the_ID() )->approved;
$hotness = 'lukewarm';
if ( $current_comments >= ( $average * 1.5 ) )
$hotness = 'hot';
if ( $current_comments <= ( $average / 2 ) )
$hotness = 'cold';
$stats = "<p class='hotness'>Hotness: $hotness</p>";
return $stats . $content;
It counts posts and comment on the first call and stores these data in an internal static variable to save time on further calls. Then it takes the comment count for the current post and calculates how much it differs from the average.
You can and should extend it. It is a guide, not a complete solution.