Originally Posted by jackal

SFO relies on its two parallel runways (which are far less than the required 4,300 feet apart) to accept all inbound aircraft, in both good and bad weather. These runways are too close to conduct parallel ILS approaches, and so when the ceilings are low enough to prevent visual approaches, the arrival rate at SFO is cut virtually in half. That's what causes the delays SFO is infamous for.

The SFO airport has actually crafted a pretty good overview of the issue (meant mostly for clueless media types to be able to try to accurately report on it, but it's helpful for all interested folks).