I guess the whole thing with user engagement is not so much that people don't know how to do something, but more that they don't want to. So people know how to comment, but prefer to vote in a poll for example.

I agree that spell checking is kind of nice, but it is not absolutely necessary and it is another option. And after all a lot of sites don't spell check - twitter, facebook, etc. I am a terrible speller, but generally a poorly spelt word can still be understood.