JS VS Server Side

When to use Java Script? When I first started Web Application programming I was told to try to use client side scripting (Java Script) only for input validation. That is because it seems to cause problems since different browsers handle it differently. But as we try to mimic more traditional systems and build everything on a web platform this seems to get kind of vague as to when to use java script and when to submit the request to the sever so that the server can handle it. I myself am still trying to come up with answer as so far it seems to be a case by case decision.