Sometimes I speak like I write, and then I catch myself, because I sound like a real nerdy nerd.
Writing should be more formal than speaking and vice versa. If I said the sentence you are currently reading out loud, it would seem clumsy and overwrought. The very word, "overwrought" is so big and heavy, it smashes into the ground like a sky-diving hippopotamus when you say it aloud. It might be acceptable in a formal speech, but even there you need to watch out for using too many big words or you just seem stuffy and boring.
But hey if I write like this it might be okay for an IM chat or talking in real life...uhhh...but it's not so good when I'm writing for something serious or even for an, um, forum.