Comic Book Resources has the most thorough of the still-emerging write-ups I've seen so far, and Comics Alliance has begun rounding up industry reactions. I expect to see quite a bit more in the coming days and weeks.
I didn't know Dwayne McDuffie the person at all, but I've long known Dwayne McDuffie the comics writer (and, to a lesser extent, the animation writer), and I spent a lot of time with that Dwayne McDuffie.
Relating the death of a real person with a real family and real friends to one's own personal experience always seems a bit selfish to me, but then, I think the fact that the passing of someone you don't really know can still affect you in some small way can be a compelling indicator of just how important that particular person is to the world. Certainly in the case of McDuffie, he was very important in our part of the world. It saddens me to think I'm never going to read another new McDuffie-written comic book, although I'm somewhat heartened by the fact that there are still chunks of his decades-long bibliography I've yet to experience personally.
Ironically, the post I had planned for today deal with one of McDuffie's works—a recent animated film he wrote—but discussing it seems pretty trivial at the moment. As does saying pretty much anything at all about comics. Other than, perhaps, rest in peace, Dwayne McDuffie, and thanks for all the great comics.