Mistakes To Avoid - Page Complexity.
It is tempting to load your pages with complex diagrams, beautiful pictures and moving objects. The more gizmos your pages contain, the longer it will take for your pages to download. If it takes more than 30 seconds, most of your visitors will have moved on. Surfers are generally after speed and not distractions.
It is the task of Web designers to optimize each Web design for high Search Engine ranking (See Bells and Whistles).