Bad Navigation
You want to make sure your users do not have any trouble finding information. It is a good practice to have a set of your main links at the top of the page as well as the bottom. It is more convenient for your users to click on a link at the bottom of your page than to have to scroll to the top of the page.
Create drop down menus. Your visitor should not have to click on one page in order to find a link to navigator to another page. Users will get frustrated and leave if they can not find what they are looking for.
Slow Loading Pages
I do not know what is more annoying than going to a website and waiting for it to load. Visitors do not want to wait for their information. We live in a fast pace world and want everything immediately.
High quality graphics and rich interactive media is the number one course for slow loading web pages. I know your website will look more attractive with these rich graphics but it is not worth it. Export your images at a lower quality to achieve faster loading pages.
Unattractive Color Scheme
This is an important and underrated area in web design. You want your visitors to feel comfortable. Do not design your website so that it is a strains their eyes. Have warm attractive colors that compliment each other well.
Do not use bright backgrounds. Having a red, yellow or green background can draw to much attention from the users and they will be distracted from what is important on the website. In addition, do not use dark text on a dark background. You should make sure your text is easy to read and is more prominent than the background.
Page Layout
Be consistent. Consistency is important. Do not make each page different. Make sure they all look the same and the navigation is always in the same location. Have the important information located at a section that is easy to find and do not forget to have a common theme throughout the site.
Sideways Scrolling
Most developers are aware of this web design malpractice. I do not see many websites where the users have to scroll horizontally. If they have to they most likely will not scroll and could miss valuable information.
Non-Cross Browser Optimization
Just because your website looks perfect in Internet Explorer does not mean it will look the same in other browsers. Each browser handles HTML Tags differently. Be careful when designing your website. Constantly check your websites progress and make sure it is compatible in all browsers.
Not Taking Advantage of SEO
Optimize your website. The only excuse for not having a search engine optimized website is laziness. Add a title to your website, META tags, ALT tags, relevant content, title tags on links and other SEO practices. Internet marketing is its own beast to tackle.
Internet marketing takes a lot of time and dedication, but making sure your website is SEO friendly is the first step to attract a lot of traffic to your website.