There are few things more important on the web than "usability," because the Internet is an interactive space and not a one-way street. You want to improve the visitor's experience, make choices simple, be pleasing to the eye and not overuse the flashy add-on du jour. In addition, your site will tell visitors a lot about your company just from the way it looks, loads and functions, before they even read a single word. The importance of creating an attractive, user-friendly website simply cannot be overstated.
For reasons that are almost too numerous to list - marketing, sales, psychology, trust building, perceived professionalism, etc. - the way your website is experienced by users should be foremost in your mind. The following eight important reminders will get you going in the right direction, but you're the one who knows your customers (or should) so the finer points of personalization and "character" are up to you.
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 customer needs: Define all the kinds of people you expect to visit your site and consider what they'll be looking for. Ensure that the navigation design helps the greatest number of people to find the most popular items in the least amount of time. Don't "bury" essential information so that visitors have to dig down two or three levels to find it.
6. 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.
8. The importance of avoiding surprises. You should use the expected, usual and standard placements for expected, usual and standard site elements. Site navigation is not something you want to be too creative with, as it needs to be immediately understandable and usable. Such consistency across the World Wide Web is actually a good thing, as it tends to make people's lives a bit easier when they feel they are in "familiar territory." Generally speaking, your various website components should look and work as people think they're supposed to.
To borrow from Oscar Wilde, consider also the importance of being earnest. More specifically, you want to be seen as being earnest, meaning that you want every visitor to understand, implicitly if possible but explicitly if necessary, that you are doing everything possible to make their site visit a simple, straightforward experience. "No muss, no fuss" is a great slogan to remember.
Therefore, rather than get caught up in profound design metaphors or using your bandwidth to display every possible website trick and/or treat, you should focus on making your site into a solution for your customers. Make it easy for them to do what they need to do and then get on with their lives. Perhaps the most important thing you can give a site visitor, then, is respect and appreciation.