It's nice to see someone stand up and say whoa. I mean my first youtube post of an arduino project I did - someone had to explain my miss-pronucenation of a word and paid no attention to the project itself. User comment boxes are not always as helpful as it might seem.
Just pointing out a nitpick by itself sounds harsh, though. Instead of just posting "It's 'you're' for the contraction of 'you are'. It's easier to remember if you think of it as just replacing the 'a' with and apostrophe.", I always try to add my positive thoughts as well, even if I feel they add very little. For example, 'Great post, I know exactly what you mean when you say some people are just overly critical for no reason. One thing, though...".
Obviously, this will still make me sound like a nitpicking asshole at times, so I try to avoid the the truly small corrections (you're/your, its/it's, grammar/mispellings in general). The key is to point out that you aren't dismissing or ignoring the rest of the work, and that the nitpicks you had were really the only part you disagreed with.