I do remember when the first Diablo came out and it already was a great game. Luckily back in the days Diablo 2 could easily become so much better than first Diablo had been, Diablo 2 set it's mark on it's own genre and as we can see, it still has it's weight.

As you said, Diablo 3 had it's burden of high expectations so even if it was good, it's not easy to reach the expectations. Most of the competitors are far away from the level Diablo 3 is, which proves it's own strengths.