Honestly, I've been coming to this site for several years, and occasionally spend a few weeks perusing the forums. Aside from the "GG is Good!" and "GG is Bad!" threads, the forum has gone through peaks and valleys of civility, same as any group of like-minded people who don't always agree on everything.
There have always been angry threads, and there will always be people who sign up/sign in just to enjoy arguing with others. The mods do a decent job of responding to extreme rudeness and boorishness quickly and efficiently, when people report the behavior.
The problem people are seeing, I think, is not looking past the aforementioned threads, and reading them as the tone of the rest of the forum. Most of the threads I've read have been more civil than on most sites, and even the GG/AGG threads are more typically civil than any discussion of the same subjects raised over Twitter or other social media.
But really, it's like violence. You are seeing it more because those threads are getting more posts (machine-gun fast posts from people looking for fights), and are staying visible longer on the main page. Outside of those threads, I'm not seeing a good deal of uncivil behavior, though there are of course exceptions.