Houston has lost so MUCH and to this day continues to lose older buildings. Right now, the River Oaks Theater (still a movie house), West Gray Shopping Center, and Alabama Theater are all in danger to make way for more typical crap (a Barnes and Noble, parking garages, expanded strip centers). Last year, the 10 story hotel across the street from Minute Maid Park came down in the middle of the night. It is a surface parking lot now (as if downtown didn't have enough already). BUT, here's a look at what I truly miss;
Old City Hall and Market Place facing Market Square
The Rossonian Apartments
The Medical Arts Building (still surface parking downtown some 20 years after it came down)
Harris County Courthouse
original Houston High School in Midtown area
The Shamrock Hotel came down in the med center. It is still surface parking 20 years later.