Have you ever been to a website that's so "creative" that you have no idea where to go or what to do? I definitely have; I've also created a few sites that I'm sure have confounded and befuddled many a user.
Most designers are artists at heart. As artists, we have a tendency to try to create things (websites included) that are very abstract, and even ambiguous sometimes.

When designing a user friendly website, though, we must fight off these urges and put ourselves in the user's shoes.

Here are a few things I've learned over the years about designing for usability. I've drawn heavily from a book called "The Unusually Useful Web Book" by June Cohen.

1) Really think about who will be using the site. For most sites, everyone from little ninos to Grandma will need to be accounted for. What browser are they using? What resolution is their monitor? All these things must be considered.

2) Make it clear to the user where they are on the site and make it easy for them to get where they want to be. A great way to do this is to use "breadcrumbs", links that leave a trail to where the user has been.

3) Make the navigation completely clear. Nothing will make a user leave your site quicker than confusing them with navigation. Users don't generally enjoy games like "Find the hidden Nav" or "Try to guess where this button will take you". Put the nav in a very prominent place and label it with straightforward names. If a button takes the user to your Portfolio, label the button "Portfolio", don't try to come up with cute or mysterious names.

4) Don't try to blaze new trails in web design with every site you create. There are certain web conventions that users are comfortable with and that will make your job much easier. For example, to get to the Home page of a website, convention tells a user to click on either a Home button or a logo in the top left of the site. This is an action that has been learned by users over time, and changing this will only serve to confuse them.
Along the same lines, if you feel the need to write instructions about something on your site, it probably needs to be redesigned. Generally, web users don't take the time to read instructions. If they can't figure out something on their own, most of the time they will just leave the site.

5)  Probably the most important part of a functional website design is the organization. This encompasses everything from main navigation down to the smallest details like spacing and text color. The planning of a site is just as important (perhaps ever more important) as the design.

Of course, these tidbits don't apply to every single situation on every single site on the web. But for the vast majority, these are great things to keep in mind. And by no means am I saying "don't be creative". Just be considerate in your creativity.