You should avoid these things when designing a website.
It is crucial to design your website correctly because it is one of those things that will make or break your site. Web developers pay attention!
Flash, Just no. Hopefully you are not using Flash anymore to design or animate your websites. Flash is quickly fading into the past because of the lack of support from many mobile devices. Cellphones and tablets are taking over the world, so if your website doesn't show up on those, your clients will leave and go straight to your competition. Besides... nobody likes those annoying flash intros anyway.
Fixed-layout sites Fixed-layout sites are sites that have a set width of a set amount of pixels (usually around 980px). These websites are fine for desktop users, but mobile devices will not play well with your fixed layout. Parts of your website will be cut off and text will be too small to read, forcing your users to zoom in to read your content. Your goal as a web developer is to make your site as user friendly as possible, so making your site fit into mobile devices will greatly improve the user experience. Responsive layouts are usually the preferred way to achieve the best of both worlds. With a Reponsive layout, the website adjusts to the browser's width with the help of CSS.
A dysfunctional Contact Page The contact page is one of the most important parts of your site. This where conversations and conversions begin with the visitors/clients. Make sure that your contact form has a receipt or thank you notice once the user sends you a message. This will assure the visitor that you have received their message and you will get back to them shortly. Another big problem is not including your contact information. Contact info is obviously very important and should be easily available to the user, maybe on the footer or header.
Widely-Used Templates or Themes With the growth of Wordpress and other CMS platforms in recent years, templates and themes are the new way to make a site look decent with minimal effort. Using the same template as others, will hinder your business's ability to stand out from the rest of the competition. These templates should be heavily customized to make the site unique and refreshing for the audience. Although these templates are easy to use, I recommend designing the website from scratch. This will result in a truly unique website that fits the business perfectly.