Great game, personally I know couple other games that are as great and not as popular. Read on.
This makes the in-game trading fun and interesting,
and online play through battle net was revolutionary.
Blizzard always takes their time on developing their game,
assuring the bestest quality before releasing the game,
same goes for this game diablo2 is very detailed and I have never encountered any glitch. (although some people dupes items I hear)
Personally though there are few other games that are as good as an RPG but not as popular as this game, and I believe diablo2 is way more popular due to the online play.
I think that other company that makes great RPGs should implement the idea of online play like battle-net and they will become more popular.
This game will remain the pioneer of online play though,
battle-net was revolutionary and brought lots of casual gamer to addiction.
The only downside I see is the PVP aspect.
After certain level (about 40) it's either you die in 1hit or you kill the other player in 1hit. There's usually no "Good fight" in pvp caz it mostly ends in a blink.