If you ignore the power of Haskell's type system, and write all your code imperatively in the IO Monad, and continue to write large functions that do many things instead of writing smaller functions that can be composed, you gain almost nothing from Haskell, and you might have been better off writing your code in another language.
Don't know about that. Even if you write everything in the IO monad, Haskell is still a pretty great imperative language and many people say it may possibly one of the best imperative languages ever written.
If you only use IO, then yes, you no longer gain the ability to reify and isolate effects as values but all the other strengths of Haskell's type system mostly remain invariant and just as useful.
the type system allows abstraction to a much higher degree than in languages with weaker type systems
That may be true, but that's simply irrelevant in this case. The type system is not being used in [1] for any interesting searches (proofs, return type polymorphism, overload resolution, etc) here outside of the most trivial of polymorphic literals. Every standard library function in use here works practically identically in any functional language, including dynamic ones such as Clojure.