I went to see The Hobbit today, and I was going to write about that, but it would only have been what I liked or didn't from a fanperson perspective. Instead, here are some of my opinions on grammar!
I am a bit of a nerd for grammar. In fifth and sixth year I spent a long time considering studying linguistics. My Leaving Cert Spanish teacher once told me that it was clear from my translations and essays that I had a very good grasp of English grammar, which transferred well in understanding the underlying principles. (Thanks, I speak it like a native!) I taught myself Latin for the Junior Cert because I wanted to understand the roots of words and a more rigid set of grammar rules. I will correct my own grammar in conversation rather than hope people don't notice, because it will irk me to leave it incorrect.
In a complete non-contradiction, there are few things that will make me negatively and immediately judge someone's entire character as hearing or seeing them correct someone else's grammar, in real life or online.
To get the exceptions out of the way: You might be a teacher, they might be learning a language and have asked you for pointers*, they might be learning a first language, they might have asked you to proofread a document. If there are others, I can't think of them at this time.
There's a line by Feynman in What Do You Care What Other People Think? that I can't find by skimming through it. He explains that in school he was a poor speller, and that it frustrated him that his teachers cared so much. If it was clear which word he was trying to use, why did it matter whether there were a few letters missing or rearranged? The lesson here isn't that, if you have a good grasp of grammar or are good at spelling, you are smarter than Richard Feynman. It's that Richard Feynman didn't have time for pedanticism where it wasn't needed, and was an uncommonly intelligent man, and these two facts had nothing to do with each other.
Convention is a very handy thing, but the meaning can usually be guessed from context (there/their, you're/your), and if someone is speaking to you casually, or chatting online, or posting in a forum, the aim is surely to communicate, not write eloquent and elegant prose, or even full words. If you halt a conversation because, and to discuss the fact that, someone has offended your sense of aesthetics, you are a terrible conversationalist. People launch into histrionics like "That hurt my eyes to read" or personal attacks like "It's hard to take you seriously when you can't even run a spell-check [in this online discussion in which many people are posting balderdash as truth because they can't be bothered to check Google]" as though they hold the intellectual high-ground. As I said, regardless of what's technically correct (the best kind of correct), the degree to which someone's spelling error or use of txt spk bothers you is down to your own sense of aesthetics, and is no more high-minded than "Can you believe she thinks that top with that skirt?" Language is for communication, and clothes are for not being naked, and bad language and ugly clothes work just fine to those ends.
It seems particularly snobbish to jump down someone's throat for a mistake in English, which is an absolute mongrel language. (Fun grammar fact! When English was being standardised, the pluralisation of 'child' fell through some crack in the system and became a double plural.)
As I said, I am a sap for eloquence, which usually includes good grammar. By all means, correct your own grammar! Let your eye twitch when someone wants to talk about "there expereince"! But say anything about it (barring the caveats above), and you are being flat-out obnoxious.
* I find it really difficult to correct someone's grammar even when they've asked me to. I worked in a language school last Summer, and when I was chatting with the Spanish kids I found it difficult to fight the impulse to speak Spanish so that we could actually have a conversation, or to correct even the most enthusiastic students, even though they were paying to improve their grammar. It's just so abrasive in every other context!
