Blizzard's vision for Diablo IV involves a "shared world." You can walk into a town and see other players, participate in open-world world events, and run into others during World Boss fights.
For players who enjoy Hardcore mode—where character death is permanent—the risks are even more severe. A disconnection caused by server issues can kill a character that a player has invested dozens or even hundreds of hours into, creating frustration that goes far beyond simple inconvenience. diablo iv offline mode