Interesting discussion. But then, I teach college writing, so my standards of "interesting" are suspect at best.

I'm with the OP. I'm sure I could be accused of "Grammar Nazism" (or is it "Naziism"?) from time to time, but I try not to be a jerk about it (unless I'm in class, of course).
Anyway, the your/you're thing does bother me, because it's a pretty obvious error - like it's/its and there/their/they're. It doesn't have me foaming at the mouth with rage, but it makes me wince inwardly a little and think, "Ooh, shoulda caught that one."
My other job is as a proofreader, actually, so I may be more aware of these kinds of errors than most. But the suggestion that nobody should care because we all make mistakes... well, spelling and grammatical errors can yank me out of the game world just as much as screwed-up textures and wonky AI. Errors of any kind remind the player that he's playing a game and - yes - hurt immersion. Not the end of the world, no - but they should have known better.
*proofreads post nervously*