This is an extremely terrible idea. I say this from experience after playing WoW, not only will the servers become way less social, it's harder to make new friends from your server since you will hardly ever party with them unless they are in your guild. This also turns many "good" players that were hiding behind an act to complete douches when they know they can get away with things like stealing loot, bashing party members non stop, and oh boy don't even get me started when you get caught with a guild party in the party search. 7 times out of 10 they are complete asses just to be asses, and almost always try to get the loot for their guildies over the "extra" player with them by all rolling on said item even though only one of them needs it. Hopefully they do not add a kick function, or add one with specific requirements at least, because that also happens a ton and totally ruins any fun of the game because of so many extremely greedy people. With that said, screw this option.
