For what it's worth, I think your analogy makes a great point, but may I make one slight alteration?
Suppose that her comments aren't things like, "You're loud and boring," or "Stop using double negatives." Suppose instead that she is walking by when she notices these family members mercilessly picking on a child, calling him names, telling him he's stupid, and making jokes at his expense. She isn't going to say anything, but finally she can't take it anymore and she turns around and says, "Hey! Why don't you pick on someone your own size, folks! He's just a kid!"
Look, the way ArielBelle made her point wasn't ideal. Yes, she should have waited to get more involved in the community before posting it. And yes, it's incredibly annoying when people don't proofread their posts or use the search feature. But I do agree that all of us could stand to be a bit more tolerant of others. There are ways to say things that are firm but polite. There's no need to ridicule others.
I'm a relative newbie here myself, but I've been doing internet discussions (message boards, Usenet, old-school BBS, etc.) as long as anyone else here. I run several sites of my own. This is a common problem across text-based communities.
The thing is, you don't know what someone else is going through. I've seen people mercilessly tease a member for their spelling or grammar without realizing that the poster had Down's Syndrome or was from another country. One of my best friends is deaf and has a terrible time with English grammar, as it's very different from the way many Deaf people think and communicate. You really have no idea why someone else says or does what they do, so it couldn't hurt to be pleasant while you point them to the search feature or another thread.
Just think to yourself, "What would Tigsmom do?" and then you'll be okay.
*sigh*... I can't believe I just posted on this thread.
-p.b.