Data and logic flow are key to the "user experience" as we know it to be today

Data and logic flow are key to the "user experience" as we know it to be today. Website user experience is important for analysts to develop a high "user return" rate. This is accomplished by a positive user experience due to positive web interface development. What website design principles do you think are most important when capturing and presenting data interactively? Why? Comment from personal experience about using or designing sites. What mistakes do you feel are most often made by designers?