According to Politico, the whole Paris Climate Treaty almost got hung up on parsing the difference between the words "shall" and "should." Apparently, US lawyers threw a hissy fit over contract language invoking what America shall do vs. what America should do (full disclosure, I am no stranger to litigious battles over the meaning of single words in the context of intellectual property disputes). But, my interest is deeper: According to Politico, "shall" has a binding, obligatory legal meaning that "should" lacks (they're fighting over modals!). "Shall" would have meant getting the US Senate involved; "should" just means namby-pamby-only-while-Obama-is-in-office. "America shall do this" was too serious for Kerry et al. and they insisted that it be changed to "America should do this." In other words, '"should" is "shall-lite." I call bullshit on that.
Consult any American or British lexicon and you learn that "should" carries the moral imperative that "shall" lacks.* Shall is more or less just a modal verb indicating future action.
Kerry et al. just morally obliged us to future actions.
* I'm still confused over whether "should" is the subjunctive form of "shall." Any thoughts?
[added] More on "should" here. And Victoria responded to me on Twitter!