Rather than a linear progression through a series of more and more difficult rooms, D II involves long walks between plot points and plenty of game time where you're running around looking for the next step in your current quest. Although it falls short of modern open-world games, it took a huge step forward for action RPGs in introducing exploration as a game mechanic.Â
Blizzard
Diablo has relatively few quests (just six per act, with only five acts), and they usually start very open-endedly. For instance, you might be told to find a character's hammer. The character has a vague sense that they left it in their forge, which was in a monastery (I always go to church to find a hammer), but you'd have to go find the monastery, then figure out where you'd put a hammer inside of it (behind a giant monster, it turns out). D II didn't hold your hand or mark your destination on its minimap; rather, it forced you to run hither and yon, exploring its rogue-like world and drawing the ire of its hordes of monsters.
One of the most surprising choices the game made was where it spawned monsters. Traditionally, monsters spawn in dungeons, just like you were taught in AP Biology. D II kept dungeons around as set pieces spread around its huge overworlds. But unlike other RPGs at the time, monsters would spawn not only in the dungeons but also in the overworld.Â
Blizzard
Since Diablo II involves a lot of backtracking or exploration in its overworld, you'd often find yourself thinking, "Ya know, I'm a bit low on potions, maybe I should head back to camp," only to be ambushed on your way back by dozens of Medusae and barely escaping with your life. The way monsters spawn sends home the message that this world has been completely overrun by the minions of Hell.