I don't see anything wrong with some commenters agreeing - if you agree you agree. What I think 'echo-chamber' means as a criticism is that commentators who don't toe some invisible line, they are banned - and I'm not talking about rude or disruptive behavior, I'm talking about simple, reasoned disagreement. I posted in a few threads on a feminist blog (ThinkingGirl) and was 'banned' apparently just for not agreeing with her in a thread that all white people are by definition racists. I was not rude. I stuck to the topic at hand. I simply disagreed and stated why I disagreed and why I, in fact, thought to label an entire demographic like that was racist because it puts race ahead of the individual.
To me, if one's ideas can't stand up to reasoned debate, that is a reason modify those ideas, not censor opposing ones. Out of general principle, I will never attempt to post at that blog again, though I did post some of my thoughts about the various topics there on my own blog. Again, anyone can ban for any reason - your blog, your rules - but that doesn't mean you are immune from criticism for doing so. And hey, I have my own platform, my own blog, so my voice continues.
Still, I think censoring for ideas is rather short-sighted - you'll get inbreeding of ideas and you will get further and further disconnected from reality. I think in some of those blogs (I've only looked at a few feminist blogs) there is a bubble of an alternate reality where it is just taken as a given that all men are evil perpetrators of the patriarchy, and everyone who disagrees with their narrow band of feminism are just enablers. RenegadeEvolotuion's blog is a reaction to that all the time - she has to put up with quite a lot that is undeserved just because of her profession.
Ideas unchallenged go stale and decay. That I think is the main problem when you censor and moderate your comments on a blog into an echo chamber.
There was also an interesting comment by someone else that linked to an earlier comment that I found amusing.