its not really cold, per se - its just cold for Californians. But its very boring, mostly the weather is monotone (in the city of San Francisco itself) - its usually very grey and gloomy. Other parts of bay area, for instance Berkeley (which is where I used to live), and more towards south, like San Jose, are actually very sunny and warm.
and yes, occasionally the city spoils us with a one day summer in the middle of the winter, and sometimes the weather is just unpredictable, but for the most part its kind of the same thing - a whole lot of monotony and grey.