Poor Contrast: Ensure website content is aesthetically pleasing and easy on the eyes.
Poor Navigation: Make sure the website can be easily navigated through internally. The user should not have to rely on google searches to locate content on the website.
Irrelevant Content: Avoid putting content on a webpage or website that is not related to the reason the website exists in the first place
Verbosity: Say what needs to be said, nothing more.
Banner Advertising: Although banner advertising is a popular source of revenue many users find it annoying and distracting.
Condescending Language: Present the material on the website professionally and matter-of-factly. No one likes being yelled at and no one wants to visit a website that makes them feel inferior or stupid.
Poor Page Organization: Ensure each page is well ordered and contains enough content sections to deliever the message without distracting the user.
Link Farms If posting links to external websites, ensure there are no more than a few per web-page and also ensure that the vast majority of your webpage is actual content. When in doubt 80/20 (80% content / 20% links) is a good fall-back ratio.
Writing for Robots Create content that human beings will enjoy reading and purusing. No one, including the search engines, wants a website designed to hall in search engine hits and not human eyes.
Failing to Validate Ensure all webpages on the site satisfy the latest and greatest requirements by using a code validator such as the W3C validator. (Link here)
Use Color Palettes: A great way of avoiding terrible contrast in development is to use predefined color palettes.
Create a Navigation Bar: Devoting a section of each webpage to a menu with links to the major webpages on the website is is practically a must.
Grouping Related Content: Make seperate groupings for related content. Using HTML5 element tags such as main, nav, or header is a great way way of accomplishing this, although using class identifiers and division elements is equally effective in older versions of HTML.
Writing Grammatically Correct Content: In addition to avoiding spelling mistakes using the proper sentence structure, punctuation, and semantics when writing is critical to presenting the content as respectable, professional, and link worthy.
Seamless Advertising: When including advertising on a webpage take extra care to ensure it blends seemlessly with all the site's other content. The advertisments should not draw the eye of the user away from the site's main content.
Display Content Courteously and Professionally: In this day and age of incredible interconnectivity it is especially important to present public content in a manner that is person independent. Following the guidelines for academic writing is a great way to ensure courteous and professionally presented material.
Cite Sources: When presenting information from different sources, whether other online resources or resources in another medium such as print, it is necessary to properly cite these sources so users can make an effective decision as to the quality of the information provided.
Properly Indent Source Code: Whether writing an HTML document, PHP, JavaScript, or another scripting, markup, or programming language proper indentation is critical to readability. Be kind to other developers, indent your code properly.
Link to Relevant Content: When looking for information on the web, interested users often ask "Where can I find out more?". Providing links to related content is a great way to enhance the user experience.
The Golden Rule: Ultimately everyone using the web, including developers, is also a user. The bottom line in design considerations is this: If you wouldn't want to deal with it, neither do the users of your website.
Poor Content Layout: Here is an example of what NOT to do when designing a user interface.
Irrelevancy: Here is direct proof of ghosts, hauntingly alive in the ethereal realm of the internet. (Three Rivers Stadium was demolished on February 11 2001 according to ballparksofbaseball.com.)
Horrid Site Aesthetics: The site's own layout is amazingly symbolic of its own place in the User Interface Universe.