Not all content is good content. There will come a time when you want to take something off of your site. If you want to be friendly with the search engines, don't just unpublish the node. Search engines are crawling through your site on a regular basis and assigning value to each page. If you delete a page, you're just throwing that value away which is a loss for your site.
Take the time to do it right and you can redirect all of the search engine goodness that your page carries to another page on your site, or even to another web site entirely.
Here's a quick checklist for deleting or moving a piece of content. Before removing the node or page:
1. Decide which existing page on your site will catch all of the traffic that used to go to the page you're deleting. Or, you may need to create one. This page should be in some way relevant to the old page.
2. Remove or update all links to the page that appear on your site. Visitors will find those broken links and get frustrated...
