If you ever have a chance to drive through downtown Detroit and look at what photographer Camillo Vergara has termed the "skyscraper sarcophagus", do it. It will break your heart. On the upside, they actually have restored two historic downtown hotels in Detroit. And in an odd kind of upside, some of the derelict skyscrapers have stood simply because no one is rarin' to take them down for new development.
The Michigan Central train station visible from the Ambassador bridge is another tragedy. I've seen photos from that one posted on UT before. And Brush Park. If we think our Victorian mansions are decaying, you should see those. A few have been restored, but so many have been taken down (and not much call for infill in that area) that the character of the neighbourhood is gone. We've lost a lot on Jarvis and Sherbourne, but there are still jewels there.