I think he means that threaded comments encourage flamewars, since they let you respond to responses, instead of to the original article.

When reddit added comments, I think that was one of the complaints. But Digg's awful "@userX..." kludge demonstrates that people will flame no matter what, but if comments are unthreaded, they'll just flame more clumsily and less avoidably.

I've had this discussion several times with one of the early developers of del.icio.us. It is by far one of my favorite web tools, and I think one of the most useful things about it is the way that it encourages a sort of conversation between its users (ie, someone can send a bookmrak to me, which of itself, begins a sort of dialogue between us, or can write a description in the notes taht I know is intended for me). What I find disappointing sometimes is that when this happens there is no way for me respond to them directly about the link (you can send messages to another user, but then you lose a lot of the context). The most obvious answer would be to allow me to comment on other users' bookmarks or "send" them a message back about the bookmark they sent me. The explanation I keep getting when I say this is that they didn't want to distract users or muddy the purpose of the app by turning it into another blogging tool. Ultimately, I think they made the right choice not to add this feature. It keeps it clean and useful while still allowing for self expression within the core of the application. I would probably like Digg a lot more if I weren't so damned distracted by all of the threaded comments that follow each post, most of which are completely useless.