[321abc]

It's not tags which would turn HN in to Reddit. It's the community. If HN's community was identical to that of Reddit, HN would turn in to Reddit (minus the tags). But HN's community is not the same as Reddit, which is why the articles which are submitted and upvoted on HN generally differ from Reddit (with some overlap, of course).

I don't see how not having tags helps anything or anyone. It just keeps the article space flat instead of organized by topic.

Of course, the articles still have different topics. They're just not easily distinguishable from one another by automated means.

[Radix]

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.

[321abc]

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.