"Please don't use uppercase for emphasis. If you want to emphasize a word or phrase, put asterisks around it and it will get italicized."
Everyone who does so feels it's justified, but the effect is always obnoxious and it never conveys more information or meaning, only an insistence that the commenter is especially important and needs to be paid attention to.
Soon, everyone will be able to contribute to unkilling good comments, so fixing these will no longer block on moderator attention. In the future, though, everyone: please send issues like this to hn@ycombinator.com. It's too random whether we'll see it on HN itself! Not to mention off topic.