Icons make composition more amicable on web page. But different icons have different influence. Let's divide icons into 2 groups: in first there would be "strict" icons, and in the second you'll see "fun" icons.

They are bright, lush and they have soft roundy forms. they have simple details, do interface more simply and positive. But you should remember the more you will have such icons, the more them will look funny and childish.

Cold monochrome or pastel tones, clear forms. They appropriate for business themes or in cases, when you need to make many icons on one page. Often strict colours show, that product is for professionals, so they demonstrate that at first is a quality of production, at least — appearance.
For example, in Apple Human Interface are recomended to use coloured, attractive icons for often using programms, and grey colours for icons, that mean programms, utilits and products for specialists. If you proect interface to MAC don't forget about it.
Any time you may compare and combinate these styles to get individual mood on your composition. For example to make simple, positive and easy page, you can use elements of fun icons. And if you want to make something serious, it's better to use straight corners and \ or grey (pastel) colours.
www.icojoy.com | Zhebrakov Andrew (Andy-S)