Well, Python doesn't have interfaces, does have metaclasses, and does allow duck typing. Python has list comprehensions, which are very powerful and don't exist in Java. Java has a rich type system with lots of data structures, and Python just has lists. So if you are leveraging what Python does have instead of trying to recreate what Java has in Python, you're probably writing Pythonic code.
But as far as OO code goes, there are certain style fundamentals that shouldn't change from language to language: you should always strive to write code that is Shy and DRY, whether you are writing in Applescript, Python, Java, or C++.
As @delnan pedantically points out, there are actually FIVE composite data types defined by Python at the Kernel level (list, dict, tuple, set, and frozenset, according to my copy of "Python in a Nutshell"). While this is true, it isn't actually relevant to the point I'm trying to make: Python builds on lists as the essential data structure. Yes, you CAN use a list as a stack, but you can use the exact same list as a queue. And then a stack again.
Java, on the other hand, has one kernel data structure (Array, according to "The Java Pocket Guide), but in general use, you can't get much done in Java without importing collections. Once you do that, you have access to a 'rich' (in which sense I mean immensely complex) type library with which to get the same functionality you had with Python's list.
Of course, both languages have classes and Java has interfaces, but while those are composite data types, they aren't really data structures in a textbook sense.
One difference being that you can't pop an item from a Java Queue, and you can't pass a Java Queue object somewhere that's expecting a Java Linked List. So perhaps by "rich" I actually mean "rigid".
So to explain what I mean by saying "Python just has lists", what I mean is that you can do pretty much everything you need to do in Python that you would do with Java Collections using Pythons List type. This single type does the work of a great many types in Java.
What does this mean for the Python programmer? It means that you can leverage the Python List type to write very pithy, direct code without the use of additional libraries--and pithiness (that is, the characteristic of conveying more value in fewer characters) is a core characteristic of "Pythonic" code.