It's easy to argue that one of Seattle's darkest days was when the Music Hall theater was destroyed downtown to make way for a parking lot.
But it's usually harder to define what buildings deserve protection. There are a few ideas in this article in the Seattle P-I.
One point in the comment thread made sense to me:
Rather our approach to "preservation", why not adopt a simple "ancient light" law. If a window has had sunlight for 50 (let's say) or more years then that sunlight can not be taken away. Period, it works in other cities.