Bad Search
Overly literal search engines reduce usability in that they're unable to handle typos, plurals, hyphens, and other variants of the query terms. Such search engines are particularly difficult for elderly users, but they hurt everybody.
A related problem is when search engines prioritize results purely on the basis of how many query terms they contain, rather than on each document's importance. Much better if your search engine calls out "best bets" at the top of the list — especially for important queries, such as the names of your products.
Search is the user's lifeline when navigation fails. Even though advanced search can sometimes help, simple search usually works best, and search should be presented as a simple box, since that's what users are looking for.
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. Bye-bye smooth scrolling. Hello tiny fonts.
PDF is great for printing and for distributing manuals and other big documents that need to be printed. Reserve it for this purpose and convert any information that needs to be browsed or read on the screen into real web pages.
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.
Most important, knowing which pages they've already visited frees users from unintentionally revisiting the same pages over and over again. These benefits only accrue less than one important assumption: those users can tell the difference between visited and unvisited links because the site shows them in different colors. When visited links don't change color, users exhibit more navigational disorientation in usability testing and unintentionally revisit the same pages repeatedly.
