Freedom of travel and inability to restrict cross(-state)-border commerce mean that whole classes of potentially-very-nice policies can't be part of those "expe

Freedom of travel and inability to restrict cross(-state)-border commerce mean that whole classes of potentially-very-nice policies can't be part of those "experiments", though, or can be but only at much higher cost than they'd be if states could control those things. Others don't make much sense at a state level, like environmental regulation—you can regulate pollution all you like, but the states up-stream or up-wind might choose "defect" on that particular dilemma and then you're screwed anyway.