For game that is about 5 years old it was more of a successor to Diablo 2 than Diablo 3 is.
See all the great features it had that for whatever reason was left out of D3.
True 3-D landscapes
No zone doors or loading screens
Side quests with rewards more than just gold and xp.
Set items and Unique items that actually drop.
A long campaign.
A "gem" system that involves DOZENS of different gems, not just 4 (charms and relics).
A waypoint and TP system that makes sense.
9 different classes and the ability to multiclass, thus making a lot of different class combinations.
Ability to assign attributes and skill points.
Elemental damage that actual does something! (Fire damage makes opponents burn!)