Seems like browser-based PUT is at once "too" generic and simultaneously not self-describing enough for a lot of input. (Should I be allowed to change my wife's facebook details?)

The protocol supports rejecting those edits, but what if I want the form itself to change in response to edits I'm proposing? (Greying out the "change password" button, or telling me "You can't change the relationship status of others!")

Stepping up from my examples a bit, it seems like you'd need a pretty sophisticated model for ACLs and almost arbitrary client side validations, which we didn't have until javascript.