The way I see it , downvoting is intended for comments which are "bad" (irrelevent , rude , incomprehensible , just rehashing what has already been said etc) this makes sense because downvoted comments are made harder to read.

Just because most people disagree with a post does not necessarily mean it is not worth reading..

Pherhaps there should be a seperate disagree button but I don't see what purposes it would serve , when I read a comment I decide whether or not I agree with it , whether other people do or not does not matter unless they are pointing out factual errors.

I'm also ok with downvoting for comments which you "disagree (factually)", aka you strongly believe the comment is simply objectively incorrect.

For example, if I made an unsubstantiated statement that "Water is composed of Hydrogen and Nitrogen", that would be worthy of downvotes.

Where that gets tricky is when someone asserts something couter to common wisdom, but attempts to substantiate it. You then need to evaluate their defense first.

Yes, that is true but in the case of someone asserting something counter to common wisdom then I would err on the side of caution and not downvote. Better to reply directly and point out why they are probably wrong if you have the knowledge to do so.

What really depresses is that they have a similar system on the comments for various UK newspaper websites (Daily Mail I'm looking at you) and you often find that the most uprated comments (which are then displayed at the top of the comments section) are arguments that allot of people may agree with but the arguments (and often the spelling) presented are just terrible and usually consist only of an Ad hominem attack on some person or organisation.

I generally find a thoughtful defense of an unpopular viewpoint a much more interesting read than somebody who thinks they are clever by posting some content free attack on something that is trendy to hate.

Ok, this rant is way off topic now and thankfully this sort of thing seems to be mostly absent from HN :)

I'd say it's worse if you make an unsubstantiated statement that goes with the common wisdom than when you go against it. At least when a comment goes against common wisdom, substantiated or otherwise, it can spark interesting discussion. A rehash of commonly held beliefs generally doesn't spark anything that interesting and amounts to little more than noise.