Well, I'm lucky enough not to have any in-laws. =P
But first of all, I heard about a report made here in Sweden a while ago that showed that conflicts with the in-laws are not as common as one would think, and especially compared to the image given by media.
I think that conflicts with in-laws are often based on the need for independence. Independence is one of the basic needs of human beeings, and when it's not respected we tend to go into "conflict mode". Even worse when the in-laws are acually trying to steal you wife or husband from you. Your independence is violated by people that hasn't violated it before. Probably the problem grow bigger if you spouse can't stand up and be independent to their parents, because then you are forced to loosing closeness with your partner.
So, my answere to you cuestion would be; anything that interferes with our independence.