I'm curious how many people started replying to you but ended up discarding it.

Same here, I discard at least two-thirds of the comments I start.

I think this indicates a higher average quality than other online comments better than most other indicators. In most places the comment would get posted anyway, but here you feel your writing as part of an average standard which is much higher.

I know for one that I also pay more attention to use of words and grammar on here that I probably would other places because that is what the standard is, that is what we all like to read and comments not at that standard usually end up down the bottom heavily down voted.

You don't see a lot of well written and constructive comments down voted even if they are arguing a alternate and contentious viewpoint.

I quite often write out a whole comment pointing out the error of someone's thinking, or just lashing out in general, then realize that I'm not actually adding anything, the person won't listen anyhow, and I'm just wasting my time, others' time, and being negative.

So I click 'back' on my browser and it goes away.

Since I learned to do that, my time on sites like HN has been much more rewarding.

I couldn't agree more. I still manage to slip in some meaningless and regrettable comments, but fortunately far less then the drivel you never get to see.