It's sometimes hard to make analogies between WoW and real life. After all, in WoW people know the world is entirely imaginary and that they can easily disassociate from their guild or the game itself, which is a bit harder to do in the real world. Nevertheless, it's a truism that there are as many ways to run a guild as there are ways to run a country.
Most guilds seem to be based on the "dictator + loyal advisors" model. The guild master is the be-all, end-all of decision-making in the guild. Sure, there can be votes or opinions expressed by the rest of the guild, but the GM is the final authority on everything. Most GMs don't have enough time to micromanage everything in the guild, so they usually delegate it to officers, class leaders, and second-in-commands. This system can be great if the GM is nice and terrible if the GM is corrupt or controlling. Many guilds have met their downfall from megalomaniac GMs or conspiring officers, although a GM who goes totally off the wall isn't going to have a guild for long. Absolute power corrupts absolutely, except apparently for George Washington.
A lot of guilds use a council system, which works particularly well in guild mergers. But although every council member is supposed to be equal, someone still has to have those GM powers. Death and Taxes seems to have come up with an unusual solution to this issue -- their official GM is the guild bank. They have several members at the second rank in their guild, suggesting that they either use a council system or the actual GM has a TON of alts.
A few guilds have moved up on the social evolution scale to become republics or democracies. Republican (little r, not big R) guilds have elected officers and GMs that rotate every time an election is held. A guild on my server had this system, and it worked great until the elections stopped and the GM started appointing officers. After that, things fell apart quickly. In a democratic guild, every member has an equal say. Much like in real life, this does not work for large groups of people. There are also some fringe guilds that use variants of other political systems, like communism (everything you've got belongs to the guild bank, and you need to spend your time farming for it) and anarchy (think most PVP guilds.)
What system does your guild use? What do you think is the best way to run a guild? Do you like being a GM or officer, or would you rather just slip under the radar?