Clayton Cramer has a cool post on building precision tools in a broken civilization using existing machined parts of known measurements, such as screws. What other devices will make useful salvage in a post-apocalytic world?
Power-generation would seem to be one of the most difficult problems, but hydroelectric dams don't require the same extensive infrastructure that coal and oil plants do, so I bet they could be kept running given the right personnel. Same goes for solar collectors and wind generators.
What about clean water? It would be tough for Los Angeles to survive without imported water.
Cell phones might be easier to get up and running than landlines, depending on the physical damage in the immediate area.
Refining oil for gasoline would be nearly impossible, especially since we ship in so much oil from around the world. The oil-producing nations would probably be able to maintain their gasoline civilization, but I doubt America could. We have oil, we just haven't drilled it yet and our production capacity is low because of environmental concerns.
With more limited electricity, clean water and gasoline, agricutlure would revert back to much more primitive -- and labor intensive -- techniques. Cities would disperse, and a huge number of people would have to take up farming. Tearing down buildings and cultivating land would be too much trouble, so most of Los Angeles would probably be abandoned, despite its ranching history.