I like that SML isn't pure, isn't lazy and allows mutation. While it's less elegant theoretically, it's eminently more practical for real world conditions where it's easier to teach and reading about.

I've wondered quite a lot why SML/CML isn't more popular than it is. If we compare it to the darling that is golang, it is better in every way except inertia (which was about the same until the past year or two).