It's easy to forget the humble beginnings of the series. When I first played the game--in 1997--I played it at a friend's house; he gave me a copy of it. When I got home, I installed it on my machine and played it for 15 hours straight. When I woke up next, I went to Best Buy and bought the game.
I mean, dang.
D2 was equally kick-ass; it came out in 2000 and I am still routinely playing it.
I'm hoping that when D3 comes out--whenever that may be--that it'll follow the trend.