I would have to go with the Alamo on this one. I have lived DT SA and currently work there, and almost weekly a visitor comments to me on how shocked they were at its location and relative lack of splendor.
Also very unimpressed with:
Las Vegas
Kemah, Texas
Grand Cayman
Mona Lisa (prior posters are spot on about the bizarre size:crowd ratio)
For what its worth, the San Antonio Riverwalk, if you bother to venture off of the man-made/commercial section (the "D"), is one of the most beautiful places I've been. Everything north of Houston street is relatively uncrowded, looks like a botanical garden, and is *free*. It's a lovely walk after a work day (assuming you are visiting SA for business). Of course the food on the Riverwalk sucks-- all food in tourist traps sucks-- venture off a few blocks and there are some fantastic finds. There are many other beautiful locations in this country but the few years I lived on this section of the river I was in awe every day of how lucky I was. (Note-- yeah you will pass the occasional vagrant who may or may not believe relieving himself in public is acceptable, but it's rare. And the park police do a damn good job of keeping the rif raff from the Riverwalk. Let's not forget, it *is* a city, and some of that is unavoidable.)