It's up to you what kinda people you play with it and how many different groups you play with... and there's always the single player to keep you busy if your friends don't want to play as actively as you do.
your right and that is exactly what i said .. the game will make a good single player not a good online game .. and with the ammount of people that play online they are really missing a huge market and fan base
It really depends on what you're looking for in an online game. If you're not looking for massive grinding, terrible randomized maps, repetition, repetition and some more repetition, and e-peen comparison for example, Diablo 2 can be seen as a pretty terrible game.
On the other hand, if you are looking for something to play with friends you know, with a hand crafted campaign, with multiple sidequests, with interesting writing, and deep, varied action combat, Dungeon Siege 3 might appear to be the better option.
See how that works? Different people want different things at different times.
I've played dozens of hours of Diablo, by the way.