I think Haskell's purity is a more humongous difference than its non-strictness, at least when you're talking about non-performance-critical applications. Lazin

I think Haskell's purity is a more humongous difference than its non-strictness, at least when you're talking about non-performance-critical applications. Laziness can produce unintuitive behavior in a program, and can make certain things harder, but purity is much more "in your face" as a programmer, and a more significant hurdle to beginners -- or at least, it was to me. I agree, though, that as the programmer becomes more sophisticated, the laziness aspect becomes a more significant difference and, ultimately, probably the biggest reason why Haskell will likely never become truly mainstream. Fortunately, there are promising languages out there which are both pure AND strict, such as Idris, Ur and Manticore.