First and foremost is poor navigation. An easy to use, clearly defined navigational structure is essential to any website. Important information should never be more than 2 clicks away and the navigation bar should always indicate to the user where they are within the site.
Another common Web Design mistake can occur from the type of computer the user has. For example a mac platform, PC platform, or even Linux. The operating systems like windows XP, Windows Vista, Mac OS 10 all contribute to possible design mistakes. One should always check their web designs on as many computer and operating systems as possible. Design can vary severely from mac to PC or from Internet Explorer6 to Internet Explorer 7. Consistent checking of ones design in a number of operating systems can ensure the prevention of future or present design mistakes that could arise.
Loud noises or music upon visiting a website is a career killer for the majority of people stuck inside cubicles. If you put a song or noise on your website upon loading it, many people will either leave your page or try to turn it off. Most offenders don't even place a proper option to turn off the sound, and that is a joy killer. Myspace pages seem to be one of the main places to see this first hand, while other sites still think it's cool to have music playing while a user visits their website. It's a common offense, and one that needs to stop immediately. Give users the option to turn on music, don't just assume that a user is going to be in love with whatever sound or music that you are trying to push on your website, or blog.
Other,avoid using lengthy flash animations/GIFs and giant graphics on your web page. Though these can be very visually appealing when used correctly, most users find them a little pass and annoying. Instead, utilize a simple, professional layout, as this looks heaps more classy and will therefore attract more visitors.
I believe that as a designer, you have a moral obligation to present your client in the best possible light that you can to his or her potential customers. Sometimes that means trying to dissuade them from those kind of decisions that they are adamantly stubborn on. Unfortunately, this is not always a battle that you can win.
Red and blue are at opposite ends of the spectrum. Try to avoid using these colours together. Red text on blue or vise versa is not good on the eyes and looks terrible. Also if your logo uses these colours try not to have them too close together or on top of each other.
If your going to have a header image on your website, keep it's size less than 1/3 the size of the whole page. I've seen some headers where I had to scroll down an entire page length to see any sort of content. Its distracting, and doesn't complement the balance rule. Also, keep it fairly simple. Have one main theme, not a collage of a thousand themes. You want your header to compliment your website, but you don't want your website to be an accessory of your header.