To be honest, if it wasn't for having "Diablo 3" as a keyword in my daily Google News list I would have never heard about DC. Advertising is currently holding back this game substantially. I never heard of the previous games either, and I love ARPG's.
I first saw Torchlight (another well done ARPG) at PAX Prime 2009, and played that game to death and it's doing very well on Steam.
Steam should be a good advertising platform for Soldak and get more eyes on DC.