Yes, I know containers is best in what it does but it is not very complete even if we compare it with c++ stl. Almost all the functionality of c++ stl have haskell implementation on hackage in some form or other but they lack good design and documentation and are not well maintained. I am not implying one package to do everything but more like a set of packages following a unified design with extremely well documented interface. It should be comprehensive and should support easy inclusion of new algorithms/data structures with community recommended implementations (containers can be  part of it). It should be well tested, optimized and ready to be used in production code. I have mentioned c++ stl just for reference but I am inclined more towards Boost like set of libraries. This is one place where I think haskell is lacking compared to other languages and it is one of the critical point in its wider acceptance.