WebProWorld has a long thread running on the top 100 mistakes people make in Internet marketing, and web design. (It's an open forum, so there's lot's of slack in what people construe as "Internet Marketing"...)

Some of my favorites so far:

1. Expecting to take over Google or Yahoo in three easy weeks... - Any company that guarentees this is full of shit. There is a method to this madness.
   
2. Content that is focused on the product instead of the customer. "Aint we great" is not nearly as powerful as "you can be even better".
   
3. Considering testing user experience and user interface design a waste of money. "Hell, I just gotta get it up. Then we'll fix 'er up fine."
   
4. Thinking that your idea is unique without finding out how unique (or not) it may be. - Spend some time trolling the web - I bet you'll find a site just like yours somewhere!