I have been a GM for about a year now. I inherited the wow powerleveling guild to avoid it being disbanded. At the time we were about 30 members and due to the previous GM's harsh recruitment policies we grew painfully slow. When he was gone I opened recruitment in a more open format and bolstered us to the healthy 150+ we have now. Over the year we have had some close calls but I am glad to say we are now stable and solid. Here are a few things I have learned.

Be as fair to everyone as you can. Resist the desire to show favoritism to friends or leaders in the guild. People notice it and if it is allowed to continue will cause issues in the guild.

Remain calm in all situations (even if you are not, no one feels comfortable with a leader that falls apart or explodes at the smallest little issue)

Watch out for drama, address it, deal with it and move on. One of the biggest stumbling blocks for a guild is drama. It can tear down everything you have spent months working for in a day. If you come across wow powerleveling drama, find out what its cause is and deal with it in a way that is best for the guild as a whole. Be wary of the few players that exist to tear good guilds down (they exist). Confront them and if you cannot come to an agreement it is best for them, you and the guild to part company.

You are not the King of the guild, its better to realize you are a caretaker that deals with the day to day crap of keeping the guild going.

If you raid USE A LOOT SYSTEM. Loot councils fail. And while I constantly remind my guild that loot is just our key to the next content level people still like it and expect to be rewarded for their hard work.

Keep it fun, many of your members play to escape the RL world. If your guild makes them feel like they have a second dismal job they won't play well, will be unhappy and it will lead to issues for you. (DO NOT THINK THIS INCLUDES YOU, BEING A GM IS A SECOND JOB. EXPECT TO GET DOZENS OF WHISPERS A DAY FOR A VARIETY OF REASONS).

Be available to your members. I have taken in and talked with even more players that have never spoken to their GM's and it amazes me. There is no better way for your guild members to know you care then to be there when they need you. I have always made it clear across the guild I have an open door policy and as long as I am not running wow powerleveling a raid they should contact me with any issues they have. This helps you keep a pulse on the happiness of your guild as well as aware of any trouble brewing.