Software development topics I've changed my mind on

From a use-case perspective, Haskell monads let you restrict where various effects happen. Some code can be allowed to do local mutation, other code can do shared-mutation suitable for (ACID-like) transactions. In Java you can do anything, everywhere (except throw checked exceptions from lambdas for some reason) so the monadic wrappers would give you about as much guarantee as a comment, and be less clear.

From a representational perspective, you need a generic of a generic to represent<M><A> and that doesn't really work. Or if you go for interfaces not type variables, you could have a Monad<A> but you don't know which one you have (List? Future? Parser? Either?). It's like representing the above as literal 'Object's, you'd be constantly casting.