Play Framework vs. the relative messiness of Yesod/Snap/Happstack/Scotty/raw WAI. Clojure has a similar (to Haskell's) story here. I'm understating it here, but this is pretty big. Most web applications are not special snowflakes, a framework to wrap-up common-case nonsense concisely is very valuable. Micro-framework stacks like using WAI directly or Scotty are good to have, but insufficient.
Specialized NLP libraries (in Java, at present) such as for medical lexicons. Might be replicable by being layered on top of existing Haskell NLP libs, but doesn't yet exist to my knowledge. Hardening up libraries like this usually takes a lot of time.
Slick is nicer and easier to get started with than Persistent. Partly due to documentation, partly because Slick isn't silly and embraces relational databases directly, partly because Persistent is just kind of hairy. Supporting MongoDB isn't a plus, it's a minus. Compromises the API anyway.
A migrations library of pretty much any sort at all.
Scala/xml equivalent i