According to Bill Widmer on one of his posts on Formula.com, a good website should excel in both form and function. This is an understatement in my very honest opinion. Without purpose, no website present a good user experience. From simple aesthetics to simple navigation cues, a good website exhibits an exquisite art, but in the most digital form. Anything else including the function and message the website is trying to convey, will be simply impossible to achieve. This is because when the user experience is not adequate, then the audience only feels pain when navigating such website.
A good friend of mine said that a website is the only thing a targeted audience sees. When not present, then nothing is achieved by having a non-effective website. Day after day, I have found this to be so true and accurate. So, next time you are designing or even planning to develop a web app, or any website for that matter, put yourself in your audience's place and ask yourself, whether what the website (or web app) wants to achieve, will be achieved.