What If the Word Trade Center Had Never Been Built?
First of all, the September 11 plotters would have probably sent planes flying into the Empire State Building and Chrysler Building. They might have survived since asbestos was still legal then and the fires would have done less damage. The effects of that change will be left as an exercise for the reader. (Fewer wars in the mideast? A Republican loss in the 2002 midterm elections? A much bigger terrorist attack a decade later because we didn't take terrorism seriously?)
Second, Radio Row would have remained. (ObSF: “Bernie the Faust” by William Tenn) Would it have turned into something similar to Silicon Valley? (I was inspired by this discussion of eminent domain in Detroit.)