With all the options available (especially in multiplayer), I think it will always be a good sandbox environment. The ultimate question is how the Survival aspect develops. This is where I think Jens has the skill that Notch doesn't as I think Jens works better towards a big picture than Notch.
That said, Jens is supposedly mainly trying to work on framework stuff, rather than add features. However, he seems to be giving us both. One of his present projects is increasing the sky space so you can build up higher, but we also got the jungles.