I would say that the most important historical development of 2013, the one that will prove most pivotal and culturally significant , is the Supreme Court decision casting out the Defense of Marriage law, which, along with various other court decisions and state laws, threw the door completely open for same-sex marriage. Never in all of human history and never in the wide diversity of human cultures, including those that have been most open to homosexuality, have men married men or women married women. Just as a matter of history and anthropology, the new legal and social acceptance of gay marriage is revolutionary and unprecedented.
What else happened in 2013 that you think historians of the next century will study? (That will not necessarily be the same as the “top news stories,” since historians are more interested in the big picture.)