I remember back in 1995 thinking that it was stupid for Java not to have generics, so instead you had to always cast Vector/Hashtable elements from Object, or implement your own type-specific container classes for every element type (and there wasn’t even a preprocessor to facilitate the latter).

Sum types I didn’t really miss, because you can implement a type-safe equivalent using the Visitor pattern, and retain an interface-implementation separation that native sum types typically don’t provide.