It fascinates me how linguistics is completely enmeshed in human identity in a such a way that it is impossible to categorize languages without tackling some thorny questions. All the topics you aren't supposed to discuss in polite company (religion, political affiliation, and race) become unavoidable when trying to figure out the relationship of Serbian to Croatian, what is a creole vs. a pidgin, and the relationship of Yiddish and Ladino to medieval German and Spanish (respectively). Fun stuff.