Yes, poor grammar irritates me...especially on a writer's site.
My pet peeves involve the examples that you have cited, and others, such as writing "to" instead of "too" and vice versa. I actually wrote a hub about this.
Also, I have seen the word "definitely" misspelled so many times that it seems the norm is to misspell it.
I had to break the news to my friend that her grammar was atrocious, and that her spelling and sentence structure lacked cohesiveness and clarity. She is in the process of writing a memoir, and I don't think even the best editors can fix her mammoth mistakes. But she is very angry that I pointed this out to her, and thinks that I am trying to kill her dream of writing her memoir.