While the English language does continue to evolve, nine times out of ten you CAN judge a book by its cover. If someone averages three spelling mistakes per sentence, uses apostrophes at random, writes "u" instead of you, and uses smileys, it's very unlikely that they will write anything important or interesting.
I have ideas as to why, but that it's true is a simple observation anyone can make by spending some time on Usenet or visiting a variety of websites.
Warning! the converse is not true: things can be beautifully formatted and correct, but still be completely uninteresting.