Today, there are only two notable buildings I can think of downtown which are abandoned/vacant - T&P Warehouse and the Public Market. However, in the past several buildings were left unoccupied. I'm to young to remember much of this, but have always wondered about some buildings which were once neglected.
Obviously, the tower stood empty and windowless for a few years after the tornado.
The webite lists the Blackstone as being empty for nearly all of the 80s and 90s.
Though it had retail at ground level, the Jarvis Building was pretty badly neglected for a while.
I haven't found a good photo, but the City National Bank building looked pretty bad for a while when it was stuccoed over.
Any other buildings which have undergone major changes?