First is the technical problem. The name "Bitcoin" is arbitrary, so you can't ban that. Rather you have to ban people using computers in a certain way. The problem is Bitcoin is used very similar to using the Internet normally, so outlawing it could make many other "lawful" Internet actions illegal.
Second would be the enormous news coverage the attempted ban would bring. It would probably elevate the level of discussion on Bitcoin bringing in a lot more voices that otherwise wouldn't have heard of it. People would start debating if they should be allowed to freely choose to use it or not, because it would still operate globally. Also, making something widely illegal, like gambling, isn't always enforced at the state level. Nevada obviously is one example of a state enacting common sense regulation and exercising its own right to governance. Similar challenges are happening in states with marijuana laws now running counter to federal laws. Choice of Bitcoin use would probably be just as contentious, and might go to the Supreme Court.
Third the resulting attention from making bitcoin illegal would increase the now officially illegal use of it. Many people trying to build constructive law abiding businesses would likely rebel and refocus their efforts elsewhere.
Last, of course, is Bitcoin operates globally, so making it illegal in one place only makes that illegality seem quite totalitarian when its use is legal elsewhere, like Finland for example which seems very much in favor of it.
Good points.