Rand from SEOmoz talks about content uniqueness and search engines. He talks about adding value to content, utilizing snippets from others work, how the Panda Update will eventually hurt your site if there is tons of duplication, and other content topics centered around duplication and uniqueness. He even answers the long running debate by SEOs that includes the question, “Is there a specific percentage that a piece of content need to meet in order for it to be considered unique?”.
Some people get lost in the minutiae of the rules around SEO, the rules around search engines and they think, “Well, this content exists nowhere else on the web. So I just took someone else’s and I changed all the words.” You have technically provided unique content, but you have not provided unique value. Unique value is a very different thing. – @randfish