This is by no means an exhaustive list, but it does cover some of the major offenses committed in web site design. There are many examples on the web that use mistakes like these, while some try to use several of them all in one web site.

1. Not Responsive

If you don't have a site that displays well on all devices, such as mobile, tablet, laptop and desktop, you are pretty much out of the game. In fact, unless you are mobile-friendly, you will now not be listed in Google® mobile searches, which will cut out a lot of traffic/customers from your site..

2. Sloooow Download

If a site takes more than a few seconds to load, visitors will not stick around. Proper optimization of images and code can dramatically increase download speeds.

3. Blinding Colors

One thing you don’t want to do is blind your customer/users. Bold, flashing ads fit into this category as well. Blindingly bright colors will bring them a good case of eyestrain. Some should state: “WARNING: Might cause seizures”. Believe me, your visitors will not be able to stick around.

4. Poor Navigation

Without proper navigation, your site visitor cannot move about your site to find the product or information you are trying to get them to look at. Navigation should be obvious and in the same spot on every page. Visitors are not going to stick around if they can’t immediately find what they came to see.

5. Plugins Necessary

Making a visitor download some plugin just to make your site work for them is never a good idea. People are skeptical of downloads that they do not initiate on their own…especially ones that are forced on them.

6. Flash Intros / Splash Pages

Ask yourself whether you want your visitors to wait several seconds for these to load just so they can get virtually no information about what you do or sell. Also, ask yourself if your money could be better spent on something that does not interfere with your customer’s reason for visiting your site.

7. Best If Viewed With…

You’ll find these little nuggets of information at the bottom of a site telling visitors the site will look better to them in, for example, Internet Explorer 9.0 and using a resolution of 1024x768. In the first place, the site should be viewable in any browser and resolution; otherwise, the web designer needs to be fired. It’s called cross-browser compatibility!

8. Untitled Pages

Imagine the search result…“Untitled”. It won’t even much matter whether you put a description in at that point. Do you think potential visitors will choose your “Untitled” page over one that has a descriptive title? Make sure you have titles for all pages and each title is relevant to the content of your pages.

9. Horizontal Scrolling

Never make your visitor scroll to the right just to read the end of your sentence. This is the epitome of bad design.

10. Awful Backgrounds

Background images can really set off a site if they are tasteful and clearly go with the design; however, one little techno-colored image tiled to create a background will scream unprofessional to visitors. Worse than that is trying to insert text over these types of backgrounds.

11. Poor Quality Images

This should go without saying and is primarily for e-commerce sites. You cannot expect a customer to buy a product from you that they can't see or can't see well enough to see the product details or can only imagine it in another color. This is not usually the fault of the designer, who can only use what the site owner provides; however, the site owner that is serious about selling online, should be serious about the product photos that they will show their customers.