Here are the five Evils. Ignore them at your peril. Chances are, you're probably not. Besides, you're under enough pressure trying to make the code rock solid. The best bet to avoid this fifth Prime Evil, and the others, is to employ the services of a good specialized Web designer early in the process.
Good design entails much more than aesthetics.
You can have the best-looking site out there, but if users can't navigate it, they won't come back. In my experience, the people who are really good at writing the code that makes the Web work and the folks who are good at keeping the interactions simple and predictable are wired differently. I couldn't begin to architect the kind of code that makes some of the more sophisticated sites on the Net run. My synapses just don't fire in that way. They know how to give the user every option imaginable but not how to guide them and promote a positive experience.