Here are some of those common mistakes:
Not Mobile Friendly"Is your website look beauty on computer screen? How does it look on mobile device?"
The answer to this will be responsive design. Bottom line is, your website need to both function and look great in any screens. This include desktop computer, tablets or smartphones (there are many screen sizes here).
Poor Readability & LegibilityThis is a crucial element of web design. Of course, a good interface design will grab the users’ attention but users have to read text to be able grasp the information they desire. Some websites use the most bizarre font styles and sizes that make reading a pain.
Unorganized Content LayoutA website’s content is what drives traffic to it.
How the content is structured is what will make it a success or a failure. Users do not read unless absolutely necessary but scan through information and pick out points of interest on a web page. Some designers just put a block of text on the web page and totally neglect headings, sub-headings, bullets, keywords, paragraphs, etc.
Forgetting Social MediaIf you fail to link with Facebook, Twitter, or any other popular social media, then you might be missing an amazing opportunity to connect with your website visitors.
Those social media boast billions of active users monthly, don't let them slip through your fingers.
Lack Of SitemapThere's no better way to frustrate website visitors than to make navigation difficult.
Include a sitemap so all visitors to your website know where to access the information they need. Especially if you are running a website with large contents and nested navigation.
No Testing Before PublishingIt's easy to make simple code mistakes on website, but unfortunately, such mistakes can prevent your page from attaining accessibility. Do testing before publishing any pages, fix them before going live. Sometimes, simple mistake might messed-up the look of your webpage.
Non-Adaptive ImagesWhy? Because mobile users are increasing, yet it is mean more visitors are viewing your website from smaller, slower, and low bandwidth devices. On those devices desktop-centric images load slowly, cause UI lag, and cost you and your visitors un-necessary bandwidth and money. Adaptive Image fixes that. It is intended to be used with Responsive Design.
FlashFlash animation might look cool in certain circumstances, but it takes quite some times to load. Not forgetting that many mobile devices does not support flash. You might thinking to have a flash splash screen on your homepage, we'll, avoid it at all cost, that's double trouble :)
We are not talking about code bugs or any technical defects, there are some common mistakes that we often did. But these mistakes are not less important. Identifying and fix them before they cause too much damage to both your site traffic and general impression.