The same old mistakes have been popping up since 1996. Although there's been massive improvement in web design overall (check out the Way Back Machine for screenshots of how web sites looked in the past), we still see the following issues on a regular basis:
Flooding your website with advertisements makes it look messy and disorients the viewer. Although ad placements may be a generous source of income for your site, make sure there is a visible separation between the advertisements and your content.
Let's face it: users spend most of their time on other websites. Having acquired some of the universal standards and habits, an individual will form certain expectations of your site based on what they've seen on other sites. Every website wants to stand out, but try to find a balance between maintaining standards and being over the top.
A website should be universally accessible by anyone with an internet connection. This includes multi-browser compatibility, as well as catering to individuals with physical and psychological differences (far-sightedness, colour blindness, poor spatial ability etc.). Users with different levels of expertise also have different requirements. For instance, novice users need guidance through interactive tasks. By not broadening access to all the various groups of people, a website loses considerable traffic.
One of the worst crimes in web design is poor navigation. It should not take more than one mouse-click to locate a business's phone number. Generally, users will visit a site with a goal in mind. If a person can not find what they are looking for, they will search elsewhere.
People should be able to figure out what your site is about in less than 4 seconds. You may have the 'coolest' website in the world, but if people don't know what it's about, your site will fail. Web design is about giving people what they want as quickly as possible in a way that they'll buy your product, your service, or contribute to your cause.