Thinking it was sad that he had to write it at all. Unfortunately, the term "mature" and "gamer" are terms not usually associated with each other. I don't say this out of hate, I say this out of experience. I'm a 40 year old gamer who has been around since day one of the Gaming Age. The garbage I've heard uttered in game chat and seen erupt on guild sites would make a prison inmate blush.
Matt writes this article out of his love for WoW, but the topic stretches across all game genres and can (and should) apply to any assembled mass of people (whether they're in a guild, a clique, a supergroup or just friends hanging out) in any game - massively or fragger.
While Matt's list is very good... I have an even simpler suggestion. A self -contained nugget of wisdom that would condense all 20 of those characteristics into one.
Treat everyone online as if they were standing next to you in real life.
What I mean by that is act and speak as though the anonymous, faceless person(s) you just dropped a racial slur on, slung a verbal barb at, stole loot from, insulted, or gave an ill-advised sexual innuendo to was their in the room with you. See, back in the day - long before PCs and gaming consoles made everyone virtual bullies and took the place of playing out in the yard with your friends - treating someone bad got you a righteous beating. There were immediate consequences for bad behavior, thus limiting the bad behavior.
The attitudes and filth spewed online and in guilds are perpetrated for one simple reason: people feel safe and secure hidden behind their monitor, far removed from the person(s) they're lashing out against. There are no consequences for their actions, thus this behavior has spiraled out of control.
Be the one that changes all that. The next time you feel the blood roiling up... take a deep breath and pretend the person you're about to unleash the hounds of hell on is standing next to you. Take another deep breath and remember this sage piece of advice: think before you act.