Give them what they are used to. Dark brown text on a dark gray background, for example, is a no-no. Don't use reversed light type on a dark background for lengthy passages. And avoid extremely busy background graphics on pages containing large passages of small text. This doesn't necessarily mean sitting down and drawing a flowchart, but have a general idea of how you'd like to set up your pages.
Gratuitous images are worthless.
Keep your backgrounds sedate and avoid mixing too many fonts on one page. Information is best conveyed when the medium is accessible and easy on the eyes. You need to consider the value of including these elements versus their file sizes and the ability of visitors to utilize them. Keep them in mind by giving them appropriate options. A good guideline to remember is that a monitor is 640 pixels high and 480 pixels wide.