Blizzard knows games. They've been making them forever; much longer than their trio of epic licenses (StarCraft, WarCraft and Diablo) have been around. It seems that they haven't just evolved with videogames, but that they themselves have somehow been involved in steering the course of that evolution. Their games, while seldom genuinely original, are perfected and polished into a refined total experience that manages to transcend anything achieved by their peers.
Put simply, Blizzard seem to have captured the "X-factor" and put it to work in their games factory.