Uhm, although I admit Haskell is a powerful language, there are quite a few things that Haskell can't express as clearly as Scala. E.g. Haskell can't express nominal subtyping.
not nominal subtyping in action? Any type you create a Monad instance for will be accepted, so subtyping. You have to actually make it an instance of Monad, it can't just have the structure of a monad, so nominal.
Right, I don't know much about Haskell (not actively programming in it), but I know that Haskell was not designed with OOP in mind and things like polymorphic nominal subclassing are not first-class there. There are also some things that Haskell's type system can't do e.g. the nice thing that Scala does with fold/map/reduce always returning the right type, even if it has to be a different type than the type of the source collection. I'd like to be proven wrong, Haskell is on my list of next languages to learn well ;)
Haskell's typeclasses (which are almost identical to traits) support polymorphic nominal subclassing just fine.
Haskell actually has whole program type inference (unlike Scala, which only does local inference), so fold/map will always return the right type as well, but you also get to not have to put types anywhere if you don't want to (including function signatures).
Scala's main advantage over Haskell is being able to call into Java.
