UX rule of thumb #3: Keep it obvious.

Nothing frustrates users more than forcing them to use a large portion of their brain power, time, and detective skills just to find out what they need to do next, or how to do a specific task they'd like to perform in a web application.

Users of your web application are users for a reason, and that is that they need a tool to get something done. Whether the task that they want to get done is for pleasure or for work, it doesn't matter. Whatever it is they'd like to get done, they don't want to spend any more time than what is reasonable. If they need to spend too much time figuring things out, guess what? Yes! They become unhappy!

This third UX design principle is perhaps the most difficult one to adhere to, but it is our responsibility as application builders to give it the attention it deserves.