And Blizzard isn't? Warcraft 2, and starcraft... If that isn't sticking to a formula...
Warcraft 2: Do you consider Super Mario Bros. to be the same as Mario 64? It's an entirely different game. Different mechanics, different feel, different style. Starcraft: Completely original franchise... not (as was feared before its release) "Orcs in Space." Are you saying any two RTS's are pretty much the same thing?
To put it another way, do you consider the Odyssey and "O Brother, Where Art Thou" to be the same thing? Do you think the latter was a good film?
Diablo was just a Roguelike game with Multiplayer, better graphics and more realtime gameplay. All of those elements had been done before on roguelikes, just not at the same time.
D&D has been around for a LONG time. So have computers. Are you telling me that Baldur's Gate was just a trivial combination of the two?
Was Ratchett and Clank a rehash one of the many other platformers? Most everything it did had been done before, at least as far as mechanics are concerned. Atmosphere was Diablo's big thing, IMO. The franchise, the feel, the context.
Hell, even for the sake of argument let's say they don't come up with revolutionary game design (which they have). They are known, both by fans and within the industry as being a pack leader. Assuming totally pre-existing core mechanics, they can add flavor where other companies rely on old, tired crap. Elves, fairies and dwarves, oh my!
Summing up: It's all in the franchise, and in a period where publishers have never been more cautious they're coupling a new universe with a new business model. I don't care if you don't like Bliz, but at least give its founders their due credit.