By the way, does Haskell's type system have the "value restriction" that you run into in ML?
Haskell's type system is practically a language itself. It has many powerful features that aid with expression:
* sum, product and generalized algebraic data types (GADTs)
* higher-kinded types
* universally & existentially quantified types
* scoped type variables
* impredicative types
* associated types
* type families
* kind polymorphism (a new feature)
* type holes (also new)
I'm not sure what you mean by "value restriction". Haskell, like ML, is not a dependently typed programming language. For that you'll want to use something like Agda. As for why you'd want a language to not have dependent types? Well, having them introduces all kinds of considerations with regards to decidability and type inference.
It can be annoying to work around this, because there are potentially ML programs that would be type-safe and would otherwise compile if they were not hindered by the value restriction.
I got my answer, though:
Ii is interesting that in ML, the presence of mutable ref cells and parametric polymorphism requires the whole language to be dominated by a "value restriction"  to ensure that the type system remains sound, whereas in Haskell, because IORef's can only be created (and used) in
the IO monad, no such restriction is necessary.