A good deal has been written, even before Shady Grove, about the limits that state law might put on Federal Rules of Civil Procedure, whether under the shall-not-abridge-enlarge-or-modify-any-substantive-right limitation in the Rules Enabling Act or under more novel theories. But there's not much out there on Congress’s ability to displace applicable state law through its power over federal procedure. According to Warren in Hanna v. Plumer, apparently Congress can displace state law as long as what it regulates is "rationally capable of classification" as procedure.
Really? Burdens of proof are rationally capable of classification as procedure. Could Congress put the burden of alleging a lack of contributory negligence on the plaintiff for all diversity cases, no matter what state negligence law said about the matter?
Can a state legislature use its power over the procedure of domestic courts to displace applicable sister state law as long as what it regulates is arguably procedural? If not, does it make sense that the Constitution gave Congress greater power over federal procedure than states have over state procedure? Or is the idea that Congress can use the Commerce Clause to displace state substantive law anyway, so what good are scruples about its power over procedure?
Also, what need was there in Hanna to talk about Congress's power to displace applicable state law? Was Massachusetts’s service rule really applicable? Would the Massachusetts Supreme Court have said that the rule applied in federal courts? Maybe all that Warren was saying in Hanna was that Congress has the power to regulate the arguably procedural in the absence of conflicting state law. (That would make more sense to me, at any rate.)