I have visited about half a dozen cities in the USA on business (New York, Atlanta, San Francisco, Cincinatti, Orlando etc)
San Francisco was easily my favorite and one of the few US cities that seems it was made to walk round. And I felt safe.
It is fairly touristy, but not such that it spoils it.
The old "docks" area has now been turned into Fishermans Wharf and some of the piers now have shops and bars on them. Picture here:
http://i5.photobucket.com/albums/y191/Hickbee/ fishermanswharf.jpg
There are some lovely old Victorian wooden houses that are everywhere as you walk around
http://sitemaker.umich.edu/ac210web1/files/vic torian_houses.jpg
It is very hilly, but that add to the charm. The trams are great fun going up and down the hills.
Here is one of the trams, with Alcatraz prison in the backgroud.
http://www.nps.gov/nero/nhlphoto/1stPlaceSanFr anciscoCableCars.jpg