1. Overloading the site: Having too many ads and images on a page will make your site distracting and unattractive.
You shouldn’t fill every empty space, having a white space or two will make a better overall impression. For example, scanning a webpage is harder if every paragraph is interrupted with ads and images. Empty areas between navigation, photos or text are visually important.

2. Using “splash” page or Flash intro: Most professional developers or designers will tell you that those pretty Flash intros are a pain. It isn’t the only reason why you shouldn’t use them however.
First, Flash animations are not SEO friendly.
Second, the Flash files are considerably larger than most web-optimized images; they will slow your website down.
Third, the ‘wow’ factor will be mostly waned after a visit or two, repeat visitors want a speedy and informative website.
Let’s say there are two vending machines? At first machine, you could just insert your coins, make a selection, and get the product. The other machine requires you to wait for an introduction video to load, play a one-minute presentation about their goods, and then allow you to make the selection. Which one would you use more often?

3. Ignoring common navigation design: Why re-invent the wheel when the common navigation design works fine. Visitors are familiar with the two basic navigation designs, to the left or across the top. Being too fancy with the site’s navigation is a quick way for turning off your visitors. It’s like if a car manufacturer decides to place the engine on top of the car instead at the front or rear. People are more convenient a recognizable pattern.

4. Having wasteful gadgets (widgets, blinking text, counters, scripts): Just because a new cool widget is released, it doesn’t mean you should use it. Widgets are only necessary if the visitors can get benefits out of it. When visitors use a site, they want to find useful information easily, having to sift through purposeless junk is the last thing in their mind. Web counters are mostly unnecessary and make your web design looks a bit dated. The best way to monitor your visitors is to use a good web analytic service behind-the-scene.

5. Using uncontrollable music: Occasionally a disruptive flash animation is accompanied with irritating tunes, but if music is needed to improve visitors’ experience, you should give them a way to disable it.

6. Using images to replace texts: Images make it possible to use shadows and 3D effects on a text; however it will lower website’s SEO effectiveness. To major search engines, images are not texts. Displaying your important keywords as images is a big mistake.

7. Using bad photos: Using crappy photos for your website is like using bad advertising materials to market your product. Imagine seeing a company’s main page filled with pixelated or stretched photographs. That just shows how a company thinks about its public image.

8. Using large images: Using massive images for a site is hazardous. If large resolution images are needed, optimize them to reduce the file size without sacrificing the quality too much. The slower your website is, the more your visitors are inclined to hit the Back button.

9. Using tables for web layouts: Tables are meant for only one thing, arranging texts. Don’t use tables to divide your web page. It is a good indication that a developer that does not have the proper skill to develop a professional site.

10. Using complex colors: A decent website should have a clean and consistent looking color scheme. You shouldn’t use more than three or four main colors. You need to use contrast, but don’t overdo it. Using too much of one or two colors can be a big visual mistake, also use white spaces properly.