Bad Search: Literal search engines reduce visibility in that they're unable to handle typos, plurals, hyphens, and other variants of the query terms. Such search engines areparticularly difficult for elderly users.

PDF Files for Online Reading: Users hate coming across a PDF file while browsing, because it breaks their flow. Even simple things like printing or saving documents are difficult because standard browser commands don't work. Layouts are often optimized for a sheet of paper, which rarely matches the size of the user's browser window.

Not Changing the Color of Visited Links: A good grasp of past navigation helps you understand your current location, since it's the culmination of your journey. Knowing your past and present locations in turn makes it easier to decide where to go next. Links are a key factor in this navigation process. Users can exclude links that proved fruitless in their earlier visits. Conversely, they might revisit links they found helpful in the past.

Non-Scannable Text: A wall of text is deadly for an interactive experience. Intimidating. Boring. Painful to read.

Write for online, not print. To draw users into the text and support scannability, use well-documented tricks:

    subheads

    bulleted lists

    highlighted keywords

    short paragraphs

    the inverted pyramid

    a simple writing style, and

    de-fluffed language devoid of marketese.

Fixed Font Size: CSS style sheets unfortunately give websites the power to disable a Web browser's "change font size" button and specify a fixed font size. About 95% of the time, this fixed size is tiny, reducing readability significantly for most people over the age of 40.

Page Titles With Low Search Engine Visibility: Search is the most important way users discover websites. Search is also one of the most important ways users find their way around individual websites. The humble page title is your main tool to attract new visitors from search listings and to help your existing users to locate the specific pages that they need. The page title is contained within the HTML <title> tag and is almost always used as the clickable headline for listings on search engine result pages (SERP). Search engines typically show the first 66 characters or so of the title, so it's truly micro content.

Anything That Looks Like an Advertisement: Selective attention is very powerful, and Web users have learned to stop paying attention to any ads that get in the way of their goal-driven navigation.

Violating Design Conventions: Consistency is one of the most powerful usability principles....Read more...

Opening New Browser Windows: Opening up new browser windows is like a vacuum cleaner sales person who starts a visit by emptying an ash tray on the customer's carpet. Designers open new browser windows on the theory that it keeps users on their site. But even disregarding the user-hostile message implied in taking over the user's machine, the strategy is self-defeating since it disables the Back button which is the normal way users return to previous sites.

Not Answering Users' Questions: Users are highly goal-driven on the Web. They visit sites because there's something they want to accomplish — maybe even buy your product. The ultimate failure of a website is to fail to provide the information users are looking for. Answering to a question is improving your website.....Read more...