There are many large books devoted to the subject. Let's just concentrate on some of the basics. Download time is one of the most important and is determined by site design, modem speed, network traffic and server efficiency. One weak link in the chain and you're in trouble.
Logical navigation is also high on the list.
Nothing is more frustrating than going to a site for specific information and not being able to find it easily. Multimedia is sort of a mixed blessing because it adds compelling visuals and audio but often requires a plug-in you don't have or initiates java and crashes your browser. Another big factor is browser and platform compatibility. There exists a vast variety of styles in website design. Ours tend to be simple and clean with an emphasis on functionality and logical navigation.