The typical user on the web has an attention span of a puppy. Don't bore them
Get to the point. Don't waste words. Like you, the user doesn't have much time. Don't risk losing them. Remember, "the only person who will read every word of what you have written is your mother. Everybody else is too busy" - Roger Black, Web Sites That Work

Never ASSUME
Make navigation effortless but enticing. Make the graphics load quickly but look interesting. You must entertain the user to keep their interest so that they stick around long enough to catch your message. Avoid overloading the page with text or graphics. Use the space wisely.

The definition of "multimedia" is simply "many medias".
The combination of available technologies can be used to create stimulating environments for users. Use the potential medias to create an environment (i.e. sound, video, animation, text, graphics ...etc)

Don't abuse the power
Too much movement and/or too many graphics can result in over-stimulation for the user. Make sure that the medias you choose to enhance your message don't drown it. "The motion is the message. The means used to communicate a message are more important and can have more impact than the message itself". - Hillman Curtis, Flash Web Design

The web is not print
The web is its own entity. Many of the fundamentals of print design still hold true, but the web has a whole new list of limitations, restrictions and issues. As well, the web has an abundance of new potentials, new territories, and new technologies that are expanding daily.

Build for the least common denominator
Not everyone has a T1 connection. Most people don't want to wait for a graphics-intensive interface to download. You may lose your audience. It is vital to know who your audience is.

Don't put flash before function
Remember to build for all potential users within your target market. Whether PC or Mac, Netscape, IE, or AOL, most of the people who visit your site wish to successfully see what you have to offer. If you sacrifice the many for the sake of some cool javascript or to cram some Flash interface in there, you will only stand to lose the interest of your visitor.

Know your boundaries
Understand the limitations. Then you will know how to more successfully "break" them.