D3 is a classic because... it sits on a lot of computers waiting to be activated?It sits there ... waiting to be activated, just like any normal video game ...
E.T. was one of the best selling titles of all time on the Atari console, yet it's the worst video game flop of all time, the best-selling video game that sucked so bad that it ruined Atari's reputation and they went bankrupt. Kids today will say "well duhhh, everyone knows video games based on movies suck hard"... well E.T. started the trend. It was the first game massively overselling through hype and franchise.Just by sheer numbers this game already succeeded.
D3 doesn't suck that much but the common impression by now is pretty much "meh, average game, would never have sold that much if it wasn't a Diablo sequel".