Haskell's advantages (and disadvantages) as a Web Framework are generally the same as in any other application domain: performance, concurrency, and correctness.
While I agree regarding correctness and concurrency, I wouldn't emphasize performance, even for Java and .NET, whose GCs are more developed, tuned and tested in the production environment than Haskell's one.
Edit: And for Haskell, achieving needed performance can be quite tricky with regard to its laziness.