In recently published video, Matt Cutts explained how Google ranks contents which do not have much back links or no back links. Matt reminds us about old days search engines when links were not part of search algorithms, when search engines used to rank contents based on the keywords found in contents.
Sometimes you have reputed and high quality contents on your website without many of the back links OR it may be that you have published new post and looking to have it ranked in search engines.
Matt says that Google when determining search engine rankings, compare the searched keyword against first appearance on page, and if it is found, this is the most important pointer. Further if it found second time on page, it is not much important, but still gives hints about page is related to that term, and more occurrence of the related terms means contents is about that topic being searched by the end user. Here Matt cautions webmasters not to do keyword stuffing as Google is so intelligent in determining content spam or keyword stuffing.
Further Matt says it also depends if the content is published on reputed domain, or previous contents published on the domain are of high quality and authority, perhaps here Google determine domain authority and reputation in terms of content ranking without having back links.
Matt also said that sometime user search such terms which does not have many search results on internet, in those cases Google also ranks pages which matches those terms but those pages does not have much authority.