A large portion of advanced Haskell type system features seem to be about emulating things you could do with side-effects. I guess I prefer Rust's approach to managing side-effects, or even just Scala's implied convention of: use 'var' very sparingly, and mostly locally. Yes, some guarantees get traded away, but so much simplicity is gained.

I'm not very experienced with Haskell, but I've written a fair bit of Scala and I've utterly failed to see the value in scalaz and similar libraries, despite trying them a few times. They always seem to add lots of complexity without a tangible benefit.

Coming at it from another angle, I just don't see many cases where I feel I have to repeat myself due to a shortcoming of, say, Java's or C#'s type system. If I could add one feature to either, it'd actually be support for variadic type parameters.