I work for a bank (not from USA though) and what I see as the toughest barrier in initiating a Bank is the massive amount of banking regulation that every institution must comply (not negotiable). Also the impact of new regulations on the systems running the business are usually non trivial. 
Missing any of these, means that you cant operate (or you can't do certain operations, generally critical).<p>As I said, the effort put in keeping with all this is very big. Usually the split is 50/50 with regulation vs new features (my view is as a software developer). The bank being global only makes things worse, as only adds more regulations from different places to equation.<p>Well that was the boring part. I think theres a place for new financial services but perhaps the way to get started is providing those new services and avoiding being a bank. As the institution adds more services and grows, it can begin to take into account the regulation little by little. 
That was my simplistic view ;)