I never said D2 was an MMO, however, since MMO(RPG)s are mostly fairly similar to diablo 2 in many ways, and simply build upon the online aspect, rather than the linear story aspect, I still compare the 2.
The reason I love Diablo 2 so much boils down to gameplay. The gameplay is HEAVILY effected by items in Diablo 2. However, the gameplay in most MMORPGs is not nearly as heavily based on items. In fact, most of them are simply "oh, you've reached level X, now you can use item Y, which is the best item for that level." Very few of them have magic effects like D2 does, and even fewer have a halfway decent magic effect system. Diablo 2 blows pretty much every single MMORPG away when it comes to the item system. Considering the skills in those games are for the most part fairly linear, don't have any synergy system, and pretty much top out with 1 ultimate skill, most of the gameplay systems in MMORPGs are kinda lacking. There's usually only about 1 build that's even worth making in most MMORPGs and there's rarely any item system that would make much of a difference to your character because the developemt team decided that skill based gameplay was a better way to go.
In short, I'm sick and tired of shitty MMORPGs, and Diablo 2 surpasses pretty much every one of them in nearly every aspect of gameplay, other than the MMO part itself.