The difference is that in Canada, all criminal law is federal. Thus, Crown Prosecutors are involved in most criminal cases. In the U.S., most criminal law is state or local. Our federal prosecutors, U.S. Attorneys, are appointed, but are not generally involved for run of the mill criminal prosecutions because those are the domain of state law.

We also use grand juries to return indictments instead of magistrate judges. In theory, this is more democratic, but on the flip side its much more susceptible to small town group think.