if code and content are the bones and brawn of a website, design should be a custom-cut, Italian leather glove - something beautiful that doesn't obscure the point of the site, or impede its ability to function.  Even better if it is nearly impossible to tell where function ends and design begins.  Skin, rather than glove.

that said, some cautions to keep in mind that will give your site a perfect fit, rather than muffling it in a frumpy mitten.

• a 'just put it up and we'll deal with it later' decision-making process guarantees clutter.
• more is not always better.
• know your business goals.  they'll serve you better in the long run than knowing which other websites you want to be like today.