The language itself isn't radically different from most mainstream Algol style languages. That's good because it makes it pretty easy to pick up. In addition to what you mention on variables vs constants, the only feature which is nice seems to switch-cases.

a. No fall through. This is a plus in avoiding bugs. This is minus if you really know what you are doing and want fall throughs b. Pattern Matching in case statements. This is bit interesting and should take care of the previous case.

Other than that my first impression is it's a mishmash of various programming languages. Everything else that they demo'ed seem to be features of XCode (rewinds, variable lifetype analysis etc..) rather than the language itself.