Philadelphia lost three city blocks for the creation of the Independence Mall, and subsequently several adjacent neighborhoods filled with architectural gems and colonial history in the neighborhoods of Franklin Square, Society Hill, and Penn's Landing. Most of the initial construction was with good intention considering the leveled neighborhoods were blighted and crime ridden, unfortunately much of the neighborhoods today (with the exception of Society Hill) are filled with surface parking lots, unfriendly windowless government buildings, and unappealing indoor retail.
Just a little of what's been lost:
The three blocks of buildings behind Independence Hall (the horizon of the photo) now make up Independence Mall National Park.Completely unrecognizable today, every building here on Dock Street is gone.
Here you can see four blocks neighboring Dock Street after they were leveled for the construction of I-95 which still serves as a barrier between Society Hill and the river. Note the hill at Front Street which used to lead down to the Penn's Landing neighborhood, now completely nonexistent. The hill is responsible for the difference in height between the west and east side of I-95, accommodated by the large staircases at South Street and Market Street.
Just north of Independence Mall the Vine Street Expressway wiped out a large portion of Franklin Square and Chinatown in the 1980s, and even more recently in the 1990s the Pennsylvania Convention Center consumed three city blocks of dense urban real estate, and in the past two years has expanded east to Broad Street taking with it two more blocks and an historic firehouse, a townhouse built in the early 1800s, the massive stone Odd Fellows Building, and illegally destroyed two designated historic landmarks on Broad Street it had agreed to preserve.
Obviously Philadelphia hasn't learned too many lessons it should have learned from the 50s. Particularly since we have a tendency to bulldoze in anticipation of projects that never come and wind up with surface lots that never seem to go away.