Hard to put a finger on it but it lacks a certain "playfulness" which real-world languages have.
It's like the difference between Lojban/Esperanto (artificially constructed spoken languages) and English/Spanish. English/Spanish are "imperfect" but their warts are the results of thousands of years of culture (and may not be imperfections after all, they may be a case of some kind of coding theory emerging organically)
That's surprising since "playful" is exactly how I would experience how I perceive Haskell. I guess it only goes to show how worthless subjective impressions usually are.
In any case, I think suggesting Haskell isn't a "real-world" language comes of a bit trollish, even though you probably didn't mean it like that.
I must agree with feanaro: Haskell IS a playful and experimental language. Its unofficial motto is "avoid success at all costs", which should tell you something about the playfulness of its community.
As for what you learn from it... you say you didn't see any gains? I take this to mean in your previous languages you already programmed in a functional style, with purity, referential transparency and lazy-by-default evaluation? I find that hard to believe :)
It's like the difference between Lojban/Esperanto (artificially constructed spoken languages) and English/Spanish. English/Spanish are "imperfect" but their warts are the results of thousands of years of culture (and may not be imperfections after all, they may be a case of some kind of coding theory emerging organically)