My favorite race that I never got to play is now 4th edition ready, allowing me to achieve new and exciting heights of non-play. Swank.
They also come with a place in the history of the new default setting, and a few more details about that setting -- specifically, how the empire of Nerath fell, and a couple of things that were going on in the decades before that. They even drop a note to explain the economics behind the warforged: Nobles could get out of required military service by paying for their production.
I kind of suspect that the setting will get a lot more defined as supplements come out and things have to be fitted into it in interesting ways. The core books play pretty fast and loose with that implied setting; it's there, but there's a lot that's clearly marked "do your own thing." I wonder how long that will last.