Having programmed in languages ranging from Ruby to Coq, for web apps and games, I feel the sweet spot is somewhere in the neighborhood of Java/C#, i.e. include generics but maybe leave out stuff like higher kinds and super-advanced type inference (and null!).

The main use case of generics, making collections and datastructures convenient and readable, is more than enough to justify the feature in my view, since virtually all code deals with various kinds of "collections" almost all of the time. It's a very good place to spend a language's "complexity budget".

I wrote an appreciable amount of Go recently, with advice and reviews from several experienced Go users, and the experience pretty much cemented this view for me. An awful lot of energy was wasted memorizing various tricks and conventions to make do with loops, slices and maps where in other languages you'd just call a generic method. Simple concurrency patterns like a worker pool or a parallel map required many lines of error-prone channel boilerplate.