1. Mixed Messages
Why do you have a website? Is it to sell something? If yes, then that should be the focus of your home page. Show potential customers a sample of what you have to offer. Make them want to look further. Very large logos and too much text detract from that purpose.

2. Unclear Navigation
Is your navigation too small? Does it stand out enough from the background? Are the most important areas of the site given the places of most importance? Navigation is your tool to get users to view your site. If you make it difficult, they won't bother.

3. Too Many Clicks
How many clicks does it take the user before he or she can actually order a product? Their patience may run out before they get there.

4. Poor Graphic/Image Quality
Are your product images pixilated, too dark, and blurry? Poor image/graphic quality will reflect on the quality of your product. Also, include the width and height of your images in your image tags - this allows the browser to account for the dimensions of the image and load the page as intended.

5. Too Much Text
Most users don't read web pages, they scan. Having too much text overwhelms them. If you must have more than a paragraph, make sure that the text contains links so that words get highlighted and the text is interactive. Of course, sites that are informational only are expected to have lots of text. Just try to break it up as stated above.

6. Never Updating Your Site
Users will feel reassured if they see you have changed content - especially on the home page. They will run for the hills if you still have events listed from 1999.

7. Spelling/Grammar Mistakes
We are all guilty of this. Spelling and Grammar mistakes cause your site to look unprofessional. Have someone else proof your site. Hopefully, you know an English teacher.

8. Using the Wrong Fonts
If the font you have specified for your text is not installed on the user's computer, they won't see it. That is why it is necessary to specify a font that is available on both Windows and Mac.
They are:
Arial
Comic Sans
Courier New
Georgia
Times New Roman
Trebuchet
Verdana

Georgia, Verdana, Trebuchet were specifically designed for the screen.

When you create graphics, you can use any font you think looks good because it doesn't need to be on the user's computer. Sometimes, especially with navigation buttons, you need to use small text. There are also fonts specifically designed to work at small sizes - Mini7 is a very popular one.

9. No Privacy Policy
Do you collect email addresses for a newsletter or for ordering? Assure your customers that you will not share their addresses with anyone. People are leery of their email addresses being sold.

10. Didn't Test Site/Don't Know Stats
Did you check to see what your site looked like in multiple browsers? What looks good on your computer in one browser can look like a mess in another browser; eg. text overlapping images, alignment out of whack, things breaking out of their designated areas. Familiarize yourself with the current web statistics for browsers. Ex. 1024x768 is about 50% for screen size, but 800x600 is still about 30% - You don't want to make your site too large for 30% of your potential customers.

Bonus Mistake
11. Using Bells And Whistles Only As Bells And Whistles
Flash is fantastic, but are you only using it to say you have a cool, hip site that uses Flash? If it doesn't add any benefit to the user, what's the point? If you are really attached to your Flash Intro, make sure you offer the user a way to skip it. They don't need to see it on their 10th trip to your site. If your site is purely for showing off your designs, and not to sell, Flash is a great way to go! See #1; know the purpose of your site.