1) Failing to spell check. If I had a nickel for every misspelled word or misplaced punctuation mark, I wouldn't be writing this now (because I'd be on the beach sipping Mai Tais from thousand-dollar bills.) People! You must check your work for errors! If you can't do it, team up with someone who can.
2) Failing to achieve an economy of words. Here's an example: "What happened was that, in order to ensure that the reader was getting the point, the writer went way far off base and overdid the whole thing." (This is a real example of someone's writing, collected during one of my writer's coaching sessions.) This could be just as easily expressed as, "To ensure the reader got the point, the writer went beyond what was necessary." One of my favorite Internet marketers, Nick Usborne, discusses the importance of short, concise sentences, especially for web copy, at http://www.wilsonweb.com/design/usborne-skimmers-…
Hard to argue with, isn't it?

You've just hit on one of my biggest pet peeves. Obviously, there's nothing wrong with wanting to target important keywords in your niche, but you should never write low quality, filler copy just for this purpose. Like I always say “What's the point of ranking well in the search engines if the content doesn't convert visitors?”

Thanks for the comment!

Excellent point. There are a lot of know-it-all copywriters who can't handle input and advice from their clients. I also think many copywriters are just lazy, and they would rather stick with what they know than spend the time truly researching the client's business to determine what strategy will work best for them.

Thanks for the mention on your blog! Here's another big rookie mistake: Assuming you know more about the client’s clients than your client does. A lot of copywriters try to fit clients into their own mold, rather than really listening to what the client needs.
So, for example, if the copywriter is used to getting response with short, pithy copy and using a lot of chunking on a website, sometimes they get in the habit of trying to write every client's copy that way. Then if the client wants longer, more technical copy, the copywriter's first response is to think “well, I'll do it their way, but the client is obviously an idiot and this will never work.”

The thing is that, surprisingly, a lot of times the client is right. Clients who have been in business for a long time tend to know their customers — and they have often attracted customers that are very similar to them. So, if, for example, the client is fascinated by the mechanics of shot peening — their clients really may be interested in that too. As hard as that is for you to believe. You may think someone would never read all that, but you may be wrong.

The trick is (and it is a very difficult balancing act) to balance what you know about best practices in writing with what they know about their clients. There is a perfect medium in there — but it requires you giving them the benefit of the doubt (and visa versa).

Wintress Odom
The Writers For Hire, Inc.
http://www.thewritersforhire.com