In this second part of a five-part series on the factors that cause web sites to rank in the search engines, we're going to take a close look at links and the age of the domain. We're also going to touch on the rate at which new pages are added, the rate at which new links are added, and hopefully torpedo once and for all the belief that toolbar page rank has anything to do with how high you rank on the search engine results pages (SERPs).
Topical Relationship of Linking Site
If the domain is trusted and the page you link is on is relevant, it can be enough to pass link power. Links from less relevant but powerful domains are still counted.
Getting a link from a niche website on your topic can help you with Google's Topic Sensitive Trust Rank and Topic Sensitive Page Rank, which require "community" links. Community links are links from websites in a specific topical community. If you can get a link from a niche website, even a new one, go for it.
Topical Relationship of Linking Page
Many websites have multiple themes, sometimes completely unrelated. Links from those websites are still highly valuable, but to have the greatest effect they must come from a topical page relevant to yours. For example, the New York Times covers just about everything. A link to an SEO site from the politics section is not going to be as helpful as one from the "Internet" or "marketing" sections.
Google has made multiple updates to diffuse page rank manipulation problems. Page rank in its simplest form did not consider topical relationships of sites, hence any high PR links with targeted anchor text were enough to get top spots. Google changed this with Hilltop and other algorithms. Links from somewhat related pages and high PR pages are still important, so by no means ignore them.
Link Popularity of a Linking Site
Page rank is a measure of link popularity, but link popularity can be different for two sites of equal page rank. One PR7 site may have 1000 links while another boasts 7000. I believe the link from the one that has 7000 is more valuable.
It's impossible to measure link popularity with Google's toolbar PR, so link analysis must come into play. Yahoo Site Explorer, Linkscape and Majestic SEO are good applications to help you learn the number of links a website has. It's possible that Google may discount half or more of the links that those tools show, so use this as a relative measure.
Link Popularity of the Site on other Topic-Related Websites
Google is moving in this direction. If you don't have any links in your topical community, it's going to be a lot harder to rank. If many websites of the same topic link to another website, it is a good indicator that the site is related to the topic.
Search Optimization 
