I'm not too bothered generally, but apostophes are easy, and so many people think you need an apostrophe for plurals eg "gym's" rather than just "gyms".
Random capitalisation is also annoying.
I like to think that I can write relatively good English, be it spelling or grammar. One thing that still trips me up are apostrophes. I'm getting better though (e.g. looking back to my journals a year ago, my spelling was quite lazy and I hate to read it these days).
I guess I didn't really care about it last year, but after becoming an active member here (since I started work coincidentally!), I have become more interested in better spelling and better use of grammar. I barely got a B at GCSE English, and I'd like to think I'd get a much better grade these days!
Back to the point of the thread... I see other's (can anyone tell me if that is the correct use of the apostrophe here?) posts and often laugh or think I'm slighty superior to them, but I think I've only ever corrected maybe 2 or 3 posts. You could say it doesn't annoy me to see other people's inaccuracies.