There are some common criteria among modern "Western" countries:
* Membership of NATO and/or European trading blocs.
* Broadly pro-US foreign policy.
* Greco-Roman based legal systems.
* Being located in Europe or North America.
* A temperate climate.
* Not being a former communist country.
However, having one or more of these does not make a state "Western". Not having one does not exclude someone.
Ultimately, it's a matter of "self-definition", of course.
I'd disagree, except for the last sentence.
. In my view, particularly the membership of NATO or EU is not a common criterium for membership of the "West". These organisations are created on the values that "the West" shares, not the other way around. Furthermore, states being commonly considered as "Western", like Austria or Norway are not part of the NATO.
Also, having a communist era in history is no criteria for "being Western" at all, as most post-communist societies like Poland, Czech Republik, Slovenia, Baltic States etc. are now definetly "Western". It is the overall history and culture, what counts. For example, Poland was always considered as the buffer against the East, thus always counted as an essential part of Europe. Russia, in contrast, was never counted to Europe until the "Westernization" of Car Peter the Great was introduced; ever since, Russia is much nearer to the West, but because of its different religion (Orthodox), history and huge ethnicities in Asia I would not count it to the "Western" civilization.
I also disagree with the criteria of foreign policy. A "Western" country is considered to have a developed and functional democracy. Thus, diplomatic priorities and relations can change with a single government change. It is natural, that states from a common civilization stand closer to each other than to states of other civilizations, but that does not exclude intracivilizational conflicts. Furthermore, most European states are now opposed to the USA; does that make them less "Western"? Or Japan and Australia more "Western"?
It is, in my opinion, also not the geography, which is important. Hence, I personally include Australia or NZ to the "Western" civilization. I think, civilizations and their differences can be described only by common and different values, history and religion. And a civilization can only exist, when there are other civilizations to which one can compare his or her own.
Oh, and by the way, most of Central European countries do not have a Roman law system, but still the one that was introduced by Napoleon.