1. The importance of focus. You need to think like your visitors do. This is key to your site's success. Your customers simply want to find what they need, make the payment and get back to real life (jobs, family, tennis, whatever). If you can make their lives a bit simpler and easier, they'll reward you for it. If, on the other hand, you make their lives more complicated, they'll "surf away" and stay away.
2. The importance of understanding the medium. You are not creating a slideshow, a YouTube video, a TV commercial or a PowerPoint presentation. You are building a website for commercial purposes. You need to provide easy, simple, clear navigation on every page, since you never know how people will link to your site and what they will see first. Visitors to your site, no matter how hard you try, will not always go where you would like them to go, or do what you want them to do. Remember that, and give them a few tools to move around the site, like a sitemap and/or internal search engine.
3. The importance of non-aggression. Most Internet users, especially experience ones, like to stay in control of their movements. Research suggest that your first-time visitors are "hunting," not "deciding," so do not make unnecessary demands for clicking, scrolling, resizing windows or anything else. Neither should you put up any roadblocks that will slow down their hunting, like time-consuming "Flash and splash screens."
4. The importance of reduced load times. Tied into #3 is the notion of your site's real and perceived "speed." Carefully consider each page element and make each one earn its place, based on functionality, not "wow" value. Keep graphic file sizes small and do whatever else you need to do to have a fast-loading, easy to use site.
5. The importance of simplicity. Flash is powerful tool, especially helpful in demonstrating things that are difficult to describe in words, but it is so pathetically overused that it has turned people off. It can be a huge distraction, too, since animation and bright (moving) colors are exceptionally hard for our eyes to ignore even when our brains want to.