Perhaps you could learn from your own advice of choosing words carefully. There may well be some valid criticism within your comments, but they're worded in an antagonistic fashion.

A prime rule to giving criticisms is to try and make it sound that, as much as possible, it is not the person's fault. It is always a problem with the thing you're observing, never the human behind it. The original comment starts off with the direct personal assertion of:

you sound kinda terrified that your new influx of users might not like this site

Which is then followed by:

It's as if you don't believe in the usability of your own product.

Wording it like that makes it sound like it's a criticism of thenewgreen as a person rather than what the post says. It is of no surprise if someone becomes defensive after that. Especially as anyone who actually knows tng would be aware that he is incredibly passionate about the site and its users. He clearly believes in the project he is part of. By making the comments personal you leave little option than for the other person to refute your remarks.

A less confrontation way to write those sentences (which I don't actually agree with) might be:

The post makes it seem like the site is scared of new users

or

The way the post is worded gives off a sense of uncertainty in regards to the site's usability

That way, you can say the same thing and it also allows the OP to look at the observations as objectively as possible without having to deal with what sounds like unfounded attacks on his character.