Why is Singleton Pattern called anti-design Pattern?

At first, Singleton was introduce in GOF legendary book as one of the creational design pattern. Singleton design pattern helps to ensure that there will only be one instance of the Singleton class, for each JVM. While the Singleton pattern may be useful for certain types of classes, e.g. constant classes, class loaders, etc., it may not be appropriate for other type of classes.

But now, it's suggested that the usage of this class should be limited to those certain types of classes, because the Singleton pattern makes it almost impossible for the class (and possibly all other classes which depend on it) to be testable. It's very hard to subclass, or to create a mock object for a Singleton class.