He was a tremendously nice guy, always approachable (I was introduced to him once briefly 10~ years ago at a bar), always willing to chat with people online, and for me it was his film and television work, especially on Justice League Unlimited. Terrible, far too young.

Well, that sucks. Damage Control was one of the best things ever, as was Justice League Unlimited.

I'm honestly tearing up a little at this.

Oh man, worst news all week. His work on Justice League animated was absolutely classic. As Joe says above, he was extremely approachable, warm and generous to fans (I've exchanged emails with him now and then) and a thoughtful creator/writer, especially on issues of race in comics.

A great tribute to him would be to watch his new (and really fantastic) animated adaptation of Grant Morrison and Frank Quitely's All Star Superman..