HN is more like a subreddit than reddit itself. Adding tags would allow the original Startup News users to sequester themselves into the article threads they enjoyed, while new people would join the community unnoticed and uninitiated. Eventually the front page would boil over with TechCrunch and Apple gossip.
Tags are bad because keeping the article space flat keeps out people with only marginally similar interests.
The question is, how much are the original Startup News users doing to keep the site pure? Clearly not enough, or this particular thread would have never been started.
The article space is flat right now, but that hasn't been enough to keep the site pure either.
I think the battle for purity has already been lost. Now it's just a question of whether the site is going to be rendered useless because the number of submissions is going to be too great to slog through with a flat article space, or if it's going to be made manageable with tags.
Tags are bad because keeping the article space flat keeps out people with only marginally similar interests.