Diablo III is a good game really.

Problem is, Diablo II was a F***ing GREAT game.

In relation to the rest of it's series it doesn't deliver in that it's separated so many gameplay mechanics and method of storytelling from the earlier entries. Small things like boss arenas and cutscenes doesn't feel like much when you're making a game (and is used to all the new Action games doing that nowadays) but if you had looked at Diablo 1&2 you'd see how the narrative was carried out.

Diablo III (which should have been marketed as a relaunch of the series instead, something like Diablo: Rebirth) is not an evolution to Diablo II: LoD the same way Diablo II was to Diablo.

So no matter sales, fact is, Jay Wilson and his team failed in his job of making a good sequel, no matter the sales.

Diablo III - good game, crappy sequel.