The biggest hurdle to deal with is behavioral change. Some people are scared to death to lose their cars. They feel as if they are losing part of their freedom. It's a historic mistrust of public transportation. I'm a huge advocate of TDM (transportation demand management) as it is what I do, but even I can understand why some don't trust it. It has not been implemented well over the last few decades in a lot of places that really need it to make daily life work. A lot of cities just threw a bunch of buses and an occasional light rail system at it and hoped it would be enough. Fact is, that is not enough. You have to be able to get people to those bus and train stations conveniently or it's not attractive enough as a commute option for people to feel they can rely on it and use it daily. That paradigm is slowly shifting and TMAs are shifting towards a more comprehensive and complete system that is both convenient and cheap for the user. The key to all of this is government monies. A full-on commitment to transportation grants and subsidies are crucial to any big city's efforts to create a complete system.
We are slowly getting there as a nation, but big oil has their hands firmly planted in too many politicians pockets. Which makes these efforts very difficult if not even impossible at times.