I agree with all of your points, except one - when I'm doing something of leisure, i.e. commenting on hacker news, I actually don't think about my grammar or spelling and nor will I begin to.
As far as I'm concerned, people pulling people up on their spelling and grammar is a step to far.
I don't think anybody at Hacker News really begrudges your leisurely writing practices. However, it seems like you feel entitled to have your opinions treated equally despite the fact that you are presenting them differently. This is probably why many of your comments are getting downvoted. I am sure that you and posters such as frevd are cool, intelligent dudes; but most members of the HN community value conventional writing style. It's not in the guidelines because it is something that comes from the members of the community themselves. By intentionally eschewing the rules of writing, you are signalling to the community that you are not willing to "meet them in the middle", so to speak.
If the majority of HN posters did not care about writing style, this situation would be reversed. But, c'est la vie.
As far as I'm concerned, people pulling people up on their spelling and grammar is a step to far.
Edit: Spelling and grammer excellence isn't in the guidelines http://ycombinator.com/newsguidelines.html