The Huffington Post recently posted a story about why many critics currently savaging The Hobbit: An Unexpected Journey will regret it in the long run. It has more to do with the fuller history of Middle-earth, of which most critics are completely unaware, and less to do with them reviewing the first part of a trilogy that has yet to play out. This article gives a run down of the pertinent bits of Middle-earth history before The Hobbit, and the time between the two stories, to more fully connect them and give a proper perspective to character motivation.
Please note that there are some SPOILERS in this article for those who have not yet read the book.