1. Using a font size or style that is unreadable.
With screen resolutions getting higher and an aging population on the internet you should be careful to ensure your pages on readable. The best way to do this is use a standard font and make it large enough for people to easily read. I recommend something like Georgia, Arial, or Times Roman for the font and at least a size of 12pt.
2. Not optimizing for search engines.
All the content you create is valuable and should help direct people to your website. Most people use search engines like Google to find what they're looking for on the internet yet so many people neglect to optimize their pages to get found. It's critical you spend the time to optimize your pages for search engines. At minimum you should know the keywords you want to target and include them in your title, meta tags, meta description, and content body.
3. Using images that are too large and slow down the page.
I see this one far to ofter with WebStarts customer's sites. They go out and buy a high resolution camera, snap a photo, and upload that to their homepage without resizing or optimizing it for the web. This leads to their homepage resolving slowly simply because the data size of the one photo is extremely large. Make sure you take the time to shrink and optimize your images for the web before you upload them to your website.
4. Not having a clear call to action.
Before you start your web design you need to know the goal of each of your pages. If your goal is to get someone to buy a product for example the most obvious think on that page should be a buy or add to cart button. Too often I go to webpages, take an interest in a product or service but then don't know what to do next. Don't let this be a mystery for your visitors. Provide a clear call to action in a place they can't miss it on your page.
5. Not putting their phone number on the homepage.
With all the time and effort your put into building a beautiful and informative website there are still those who'd prefer to simply call you about your products and services. So don't hesitate to put your phone number in an obvious place right on your homepage. Chances are you'll be able to sell your customers better over the phone regardless of how much information you provide on your free website.