For me, the biggest issue to learning Haskell is once programs progressed to a certain level of complexity, the sheer amount of types that got thrown at me made my head hurt. I could deal with pure functions, IO, monads (I even like monads), even mutable variables and arrays weren't too bad. But the types, so many types and types of types... It seemed every value that was meant to represent anything had its own type and reasoning about what function to use on what type was too much.

Maybe I'll revisit it one day, but that was my pain point...