What is wrong with java's implementationof the observer pattern?

Informing myself about the observer pattern I stumbled across quite a few statements that suggested to make a custom implementation of the pattern without specifying what's really wrong with java's implementation. Is it because java.util.Observable is an abstract class instead of an interface? Yet, as far as I can see it allows for both a PULL and a PUSH mode. What could I do better in a custom implementation?