George Bernard Shaw was also a theatre critic, maybe even before he was a playwright. I could be mistaken, because there were a few of these guys who were great playwrights but also acerbic theatre critics themselves, but I think it was Shaw who was watching a particularly dreadful play one time, and he leaned forward, and tapped a woman on the shoulder right in front of him, and said, "Excuse me, madam, but would you be so kind as to put on your hat."
Many different people have been attributed with the long letter quote. Abe Lincoln. Mark Twain. Blaise Pascal. I have no idea who said it first.