I believe that the killer app for the Apple Watch will be authentication. And therefore the success of it will depend on a wide adaptation of all locks to support it, exactly like Apple Pay needs to be widely accepted. In both cases the watch is easier to use, but they need the iPhone as a steppingstone to reach the critical mass to convince people to make the effort to support it.
And this means that the watch will be likely off to a relatively slow start, until in a few years the ecosystems have grown enough for it to displace wallets and keys.