Today I realized that I'm part of the "old guard" of blogging because I remember a time when blogging was so new that very few sites had comments (it seems like MetaFilter was one of the first few?) and after a few years when they started to become commonplace, people were generally decent to each other because it was very literally turning a blog into a face-to-face conversation.
But I think the root of the problem (described in various media outlets over the past year or so) of snarky, or mean-spirited, or generally unhelpful comments becoming the norm has to do with the distance we've achieved from those original link-and-essay heavy blogs.
I have a feeling that if you've only seen blogs in the past five years (which is probably 95+% of people reading blogs today) you consider comments to be de rigueur and they are entirely divorced from the original concept of a conversation between the reader and the author of the original post. It's not an intimate conversation, it's just another content management feature available to you on the web.
This has a de-humanizing effect that I'm seeing play out more and more often in the weirdest places. People will post about their idle curiosities on their personal blog ("Why does x happen when I do y?") and instead of seeing friendly answers I would expect many years ago, I'll often see someone early on read into the question and assume all sorts of accusations ("well, maybe it's because you are a, b, and c, and everyone knows it!") and watch most followup comments start from there and go into darker directions.
It's tough because I love blogs and I love comments in blogs, but I'm starting to think there's this "new generation" that has grown up online only knowing blogs as having snarky comment areas and never realizing it used to be a personal, intimate space where you'd never say anything in a comment that you wouldn't say to a friend's face. Also, know that I mean "new generation" in a way where age of person in it is irrelevant. You could be 50 years old and started reading blogs last summer and I'd put you in that group.
Of course, I could just be talking out of my ass, old people tend to do that...