Many commenters responded with passionate diatribes against a variety of language errors. Here is the response I added to this discussion, slightly modified here:
I must admit, I'm a bit taken aback by the level of ire associated with some of these comments.I once had an excellent writing teacher who said, "The purpose of writing is to communicate clear ideas clearly." Thanks to him and others, I have developed the ability to write clearly. I have mastered the "rules" for different kinds of writing, from business writing to academic papers to grants. This is why I offer my services to others.
I have a Master's in English and I'm a college instructor of English. I'm also a professional editor. I think we take grammar too seriously.
I'm proficient in spotting and fixing grammar (and other writing) problems, so I use that gift to help others. (I actually enjoy it - guess I'm just an academia nut. Ba dum bum.)
However, I don't think this gift makes me superior to the people I help. It's a gift; I didn't have to work hard to discern the difference between 'your' and 'you're'. The people who DO have to work hard at this and then finally master it are the ones to be admired.
Language rules are arbitrary and changeable. They simply constitute a code which is different for different communication situations. I tell my students this; they love it. They can then embrace learning grammar as simply learning a code for a certain kind of communication.
Grammar Nazism does not serve the learning of grammar. Quite the opposite.
However, I am not and never will be the Grammar Police. Think of me as more of a grammar mechanic. I won't judge your writing for its problems, but I will repair it and point you to the things you can do to keep it running smoothly.