I don't know, look at how GvR treats lambda: a weak version and he still tried to back it out of the language at one point. Even when benevolent the judgment can stumble -- hell, McCarthy still does not like sexprs! :) The good thing about Arc is that it has decent macros, so not even PG can dictate to users -- we'll just MAC an end run. :)
4 is crucial in my mind. You cannot let the language have more power than the programmer. Also, it lets you build more consistent programs instead of needing your own gets and sets all over the place. This would make it a huge win in decreasing tokens a program has to remember and grok about a new program they are reading.