Why is it such a big deal to use ALL CAPS?

It's a way to draw attention to the text, i.e. the equivalent of "hey, look at me, I'm more important!" and thus bad etiquette. Instead, let the subject stand on its own merit. The same goes for posting with excessive punctuation or special characters. Simple as that.

Btw, we have a sticky thread dealing with questions regarding forum guidelines and moderation. Try use that one next time. Please.