Avoid the trappings of an in-game structure all together.
In this concept, you avoid the fixed barriers of a guild. Friendships are no longer defined or destroyed by inclusion in an arbitrary game mechanic. A "lite" version of this idea is the alliance. While people might have to join a guild to join the alliance, they can join (or create) a guild they like, with people they like, and join an alliance that provides an over-arching unity to meet a larger goal. Yet, this doesn't solve all the issues with management. You have to join a guild, and that guild will need to be managed and controlled. Most alliances still have rules for participation and to manage shared resources, such as the alliance chat channel.
The Leftover's concept is a good implementation of the "No-Guild Guild" idea. There is no "guild", only charters. Charters are for accomplishing goals. The charters function as limited guilds. They have defined goals, there is no in-game manifestation of being a member, and no expectation of being included permanently. As a matter of fact, the charters are obviously not going to last as long as the people will play the game. Charters are designed to tackle specific content in the game of WoW and when the person running the charter is done, the charter might just go away. No harm, no foul. The one thing this idea really delivers to the table is the broaching of expectations. No one joins the charter with the expectation of getting lots of guild services. And no one running the charter expects people to behave like they are members of a guild. And, if you don't want to be in a guild at all, a charter is still viable for you to join or run.
Obviously, someone is still running the charter and doing the work of putting together a raid in WoW, but this is a lot less work than setting up web services, managing guild membership and interpersonal problems. In the end, a charter leader would most likely be doing this same exact work in a guild, and be called a raid leader. But now they have a wider pool of participants to choose from and people have a wider pool of raid leaders to follow. No one is limited to the pool of players in their guild alone. At first glance, you might think this is opening up this aspect of the game to more people might lower quality of participants. But, since the charter leader and the participant are free to do as they please, they can exclude problem players from who they have to interact with. Charter leaders can opt to not invite people they think are problems and participants can not join groups run by people they don't like. They are not playing out of a pool of their guild, and thus forced to decide between their friends and their game play expectations.