Contrary to popular perception, much of LA is quite dense and a hell of a lot of people use mass transit. And if the mayor gets his way, more better transit is coming. I think two things make it difficult for LA to be a great transit city. The first is simply the psychology of the place. People really don't think anything about driving an hour in any direction. It makes the set of potential places - retail, friends, whatever - one might visit quite large. The second is that while LA is actually quite dense, in much of the city it's a very even density. Light rail and subway systems are more useful if things are clustered around stations. With the trains, development patterns will likely change if the local authorities let them, but that takes time.
I guess there is a third thing, that despite having all that lovely weather people are strangely allergic to walking.
