I have a problem with his extensibility critique. I don't agree that having generic types are good for the language. The problem in using polymorphism is that your users are going to abuse polymorphism. There will be class hierarchies that will confuse and abstract from the algorithm at hand. You will be mutating and mangling objects rather than dealing with the issues at hand.
Reading a go source file is like following a for loop. It's quite technical. Reading a Java source file can be a case of trying to figure out the high level abstractions of the program.
The genericity that is being talked about here is parametric polymorphism, not subtype polymorphism. Parametric polymorphism doesn't lead to confusing class hierarchies. Parametric polymorphism doesn't involve inheritance, so it doesn't lead to any class hierarchies.
Reading a go source file is like following a for loop. It's quite technical. Reading a Java source file can be a case of trying to figure out the high level abstractions of the program.