Diablo IV is Open World: How Does It Work?
The announcement that Diablo IV would be an open-world game came as a bit of a surprise to me and was somewhat worrying. While I'm still not completely sold on the idea, the details on how it works are quite reassuring.
The general idea is that player interaction with people outside of their party should be relatively rare outside of specific areas; you're not going to be running into other players at every turn. Instead, Blizzard's Diablo team is looking at making random player interaction largely fall into three categories: conflict, cooperation, and social interactions.
In towns, strongholds, and other 'safe zones', you can interact with other players and are likely to see them on the regular; this is likely going to be an excellent hub for trading items (trading has been confirmed to return, though with limitations).
Cooperative play will sometimes come in the form of world bosses and large scale random events found while wandering each zone, The bosses are huge and deadly, taking a large number of players working in concert to defeat.
Player vs. player combat has also been confirmed to be in the game, with no specific difference being made for balance between PvP and PvE; everything works the same with no adjustments, and balance is being considered for every option in how it affects both elements.
For those that like to play solo, however, fear not: Campaign zones are private until you complete the campaign objectives for that area, leaving random player interactions as something that only occurs on revisiting old areas. Dungeons are always private and can only be accessed by you and your party.
This sounds like an exciting way to handle things, and will hopefully give players more reason to revisit areas that aren't dungeons to experience the unique content in each zone.