Tonal shifts from one block to another is a phenomenon even long-time urban dwellers will be hard-pressed to explain.
How does North Beach "just feel different" from adjoining and amorphously defined Downtown, Chinatown and Fisherman's Wharf that there isn't any mistaking them? If you figure this one out, please make Contact :)
San Francisco is no exception to this urban mystery - the tastes, textures, smells, sounds and colors are highly variant from one neighborhood to the next. To appreciate them fully, walk or ride.
Philosophy aside, most of San Francisco is very safe. If you feel frightened, focus your gaze and walk quickly - and remember, the odds are very good.