Similar to the history of your site, the history of the links to your site have gained importance. As was noted in Google's patent application #20050071741 patrician, Information recovery based on historical data, links, like sites, gain weight over time. This point was further clarified in the most modern update as sites with ancient links gained strength even as sites with many new links did not see earth-shattering ground gained. The sandbox on links is carrying out in fractions in that after a phase of time a link will gain part of it's weight, after a bit longer, it will gain more, etc. (the exact length of time is of itinerary a closely guarded secret by Google and likely change as their algorithm does). These funds your link building today won't create any substantial effect on your Google rankings until months down the road.
Additionally, the relevancy of links to your site is still important however Google's ability to conclude relevancy appears to have improved. Pages no longer have to contain the exact keyword phrases to survive relevant but rather have to be from related industries. For example, a link to an SEO site from a web devise site would be considered relevant even though the keywords on the page are not specifically related to SEO.
Natural links have gained weight over unnatural links. Links that are contained within contented areas of a page will be weighted more strongly with the purpose of links that appear alone or in a directory-style (like reciprocal links pages) as they are measured more natural. while you are having links built to your site try to get them placed within the content (within the description segment of your reciprocal link for example). Also, in link building you will want to insure you're varying the terms of your anchor text. Creating hundreds of links with identical anchor text will hurriedly be detected as a link building effort (i.e. not natural) and thus will carry little load. Different anchor text for your links will appear more ordinary and thus will have a more affirmative impact on your rankings.
