Multiplayer

The multiplayer game is what the design team feels is the core of Diablo III. They are designing D3 to be most fun in cooperative play with a friend, PvM party, and creating game features to function best in that setting. Gold is shared, items aren't fought over since each character only sees the items they can pick up, party joining is streamlined, in-game hostility is removed, and more.

The Party System

There is no more party system in D3, at least not like there was in D2. In D2 players who joined a game were neutral, neither friend nor enemy. They could become enemies by clicking the hostile button, or become friends by joining a party together. Once they were partied, they shared experience and gold. Partying was optional though, and parties could be dissolved or abandoned.

In D3 there is no more party system, since the hostility option has been removed. (There will be dueling and PvP in special games for that purpose, but it's not going to be an option in regular, PvM games.) The D3 Team found that it was pointless to have a additional party system menu with invitations and joining required, so they simply removed that option. All players in the same PvM game in D3 are in the same party, by default.

No More Hostility

One big change to multiplayer games in D3 is that there is no nonconsensual hostility. There will be dueling and PvP in special games for that purpose (no details have yet been revealed), but there is no way to "go hostile" in a regular PvM game.

The D3 Team is aware that some method will be required to control or remove or avoid grief causing players (who could spam the game, or lead monsters to trap waypoints, or other annoying behaviour, even if they can't actually go hostile or be gone hostile against), but they've offered no suggestions about that yet. It might be possible to boot players from a game by majority vote, or the game creator might retain that right, but we'll have to wait to see what solution the team offers up.

PvP

There will be PvP play in Diablo III, but not in normal games. PvP will exist in special games, arenas or dueling games that the team has not yet released any details about. See the PvP article for more details.

Individual Item Drops

A major change/improvement to party play in Diablo III is the fact that items will no longer be fought over. Characters in D3 only see items that they can pick up; there's no more ninja looting teleporting Sorceresses, no more melee specialists grabbing all the goodies, etc. In D3 when items are dropped, they can only be seen by the character who can pick them up. Most normal monsters will only drop one (if any) items, and drop rates per player should be about the same as they were in D2, although on the whole more items will fall. Boss monsters will drop multiple items each time, so that every player in the game gets something.

It's not yet determined how much item drops will be customized; in theory a Barbarian might find a wand and an orb, while the Witch Doctor got an axe and heavy plate armor. Or the D3 team might make drops more appropriate for the character who sees them; no class-specific items for other classes would drop for your character, for instance. As of BlizzCon 2008, the D3 team talked about item drops being randomly distributed, so there might not be any customization of appropriate drops at all.

One key to this system is that while dropped items can only be seen by the character they are dropped for, any item dropped by a character can be seen by all other characters. Jay Wilson has often mentioned the phenomena of players reaching a certain point on a level when everyone's inventory is becoming full. By tacit agreement, all the players pause for a moment and drop all of their unwanted stuff in a sort of free yard sale, allowing each other to pick through the stuff and take whatever they might want.

The Importance of Multiplayer

Here is a quote on Multiplayer development by D3 Lead Jay Wilson:

AG: I know that in StarCraft 2, a lot of the development was focused around multiplayer - a lot of the game design and the development being done was on multiplayer first. Is that something that you guys do with Diablo as well, or is a bit more focused on the single-player or a bit of a mix?

Jay: For us it's a little bit more focused on the singleplayer, it's not quite the same as StarCraft. I've played RTS before and I've seen people do it both ways and I seem to prefer the method where you work on multiplayer first, because you're really establishing the core game mechanics.

For us, a lot of the core game has actually come out of singleplayer, but what we've done along the way, is we've always had cooperative play enabled. So people play cooperatively all the time. Any time anything does work for cooperative play we fix it right away. So any mechanics that don't work, any kind of balancing or tuning that needs to be done, we try to do it simultaneously for both – but we do focus on single-player more.