This might be a philosophical kind of question, but I believe that there is an objective answer to it.
If you read the wikipedia article about Haskell, you can find the following:
The language is rooted in the observations of Haskell Curry and his intellectual descendants, that "a proof is a program; the formula it proves is a type for the program"
Now, what I'm asking is: doesn't this really apply to pretty much all the programming languages? What feature (or set of features) of Haskell makes it compliant with this statement? In other words, what are the noticeable ways in which this statement affected the design of the language?