Currently everything you have is yours. If you have it, you control it. With the exception of your professions which are controlled by the group you're in if you're an enchanter. This promotes a strong concept of ownership. There is little which is truly owned by a group.
Guild banks have some gold or mats or a few BoEs, but they don't control all that much. They don't control anything that anyone uses. By that I mean, someone can use items from banks, but once used they are no longer in bank control. They either soulbind or are consumed.
WoW has almost 100% private property. Individual property.
What would happen if there were items controlled by guilds? Yesterday I briefly, and unfairly negatively, suggested that legendaries be Bind on Guild. They are made by guilds, so it makes some sense that they are controlled by guilds. I thought it would force the player who was given the item to be stuck with the guild or lose it.
I didn't consider that the concept of item ownership might change. If legendaries were BoG, then no single person could feel ownership. They would think about them differently. I suspect the item might come to be viewed less as an item rather than as a buff. By that I mean people would have their normal weapons and the guild might decide to temporarily provide the legendary to a player similar to a single-person buff, such as vigilance, but much more powerful. The weapon would be only temporary to the person, so they could not develop such an attachment.
However a big problem remains: guilds are dictatorships. Ultimately everything is owned by the GM unless people can grab it if they open up bank permissions. This means that no one would own a legendary, except the GM. I don't know how this would turn out, but it could get ugly if a guild seems items as belonging to the guild while the GM thinks they belong to him.
Would guild legendaries lead to a sort of pseudo-socialist dictatorship in which the government (GM) controls all in the name of the populace? I doubt this will happen commonly. A state can enslave and imprison and restrict its people. People can more out of guilds that are undesirable. Attachment to BoG items may encourage people to stay, but if the handling of BoGs is not acceptable to them, that attachment may be insufficient.
Moving forward in Cataclysm, I think BoGs would be an excellent addition.
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