You're not E.E. Cummings. He deliberately eschewed punctuation as part of his poetry, exploring possibilities. HN comments are not usually poetry in that sense. The point is to communicate ideas efficiently to others.
We capitalize the first letters of sentences because it makes it easier to see where one sentence ends and another begins. The period at the end of the previous sentence contributes to the same purpose. A period is pretty small, though, so a capitalized letter helps it out a lot.
Breaking this rule "just because" is lazy and requires more effort by the reader to read your text. Similar to using "u" and "ur" (on a non-phone device), it says "I can't be bothered to pay attention to detail."
When it comes to writing, I absolutely judge based on appearance. If you can't bother to write correct sentences, how can I know you'll bother to account for spending in detail? Or cover all the cases in error checking code? Of course, the way one writes is just one facet of many, but it contributes to the overall impression.
on the capitalization front, i hear you saying that a lot of people on HN are like the worst kind of VCs, the ones who will judge an entrepreneur based on appearance. disappointing.
the "break the rules, just not the ones that matter" quote is from pg in "What we look for". i like your reworking better (although it's got the same challenge about subjectivity -- who decides what matters?) this isn't a rule, though, it's more like a norm. from what you've said, suppressing my personality and adhering to the norm may make me more popular here. is that how most people see HN?