You know the wicked (and award-winning) opening credit sequence? And each castle or city is built up from cranks and such? Harrenhal doesn't do that! When they've showed Harrenhal in the opening sequence, it's been in its ruinous state and the camera just pans around it. I don't know how this completely escaped my notice before!
Do you think it's as simple an explanation as "Harrenhal is a ruined castle while the others are alive and still changing", or do you think there's something deeper going on here? After all, Winterfell was sacked but it still gets "built" (it's just a little smokey, it's still good! It's still good!). I know Ashaya and Aziz have written an essay on Harrenhal, and I doubt that this featured in their analysis but I'd love your guys' input as you've clearly put a lot of thought into the castle.
(As an aside, a Harrenhal episode of HOW would be pretty cool!)