Abusing CSS CSS can, is, and will be abused. As CSS becomes more widespread, it will be used to create ugly pages. You don't have to define styles for all the tags you're using on your page or site. I've seen some great looking pages that have styled two or three tags, or even just one. Going with gaudy colours and fancy typefaces will detract considerably from the look of your page or site, and can make it unreadable.
Complex layouts are fine, as long as they're pleasing to the eye.
Try to maintain a balance between form and content. Regardless of what you may have been told, content is the more important of the two. Your worst mistake is to create a page completely dependent on styles - one that uses your own styles and nothing else. When viewed in a browser that doesn't support style sheets, such pages appear to contain only blocks of text broken up by the occasional graphic, list or table. Users won't be able to differentiate between sections on a page.