There is a very important article out in SEOMOZ surveying what "expert" SEO practitioners opinion on what makes a page or site rank well. It is mostly important to you if you have been listening to Voodoo practitioners, and there are a lot of them out there.
I will eventually get around to discussing each factor, but in general:
* The highest ranked factors are certainly valid for Google.
* What doesn't matter for Google at this point just doesn't matter. If someone tells you they get all their traffic from Yahoo and MSN search engines, it doesn't mean they are experts. It means there is something really wrong with their optimization strategy.
* From eyeballing it, anything given less than a rank of 3 out of 4 in that article is probably a waste of time and might be harmful. These get into the realm of Search Engine Optimization (SEO) Superstitions.
* What is basically a test of the expertise of the "experts" is given in one of the Pie charts at the end of the article. About 32% of the experts said there is no Google Sandbox (new sites are not ranked for a long time). Find out who they are and do not use their services. There is no Santa Claus and there is no Sandman, but there sure is a Google Sandbox.
* Another pie chart illustrates the naivete of the people making the survey. It asks which factor is most important for "Google Rankings" - It doesn't differentiate between overall ranking of a site and ranking of a page for a keyword. It asks if "Authority" of the domain is most important or Keywords on a page or backlinks (extermal links) to the page. Most people chose Authority (= Google PageRank) of the site. I would have to say "depends," though if pushed to the wall I would say external links to the page with the right anchor text based on empirical data. Of course, it depends how many links and who is your competition. Wikipedia pages are always going to rank high. But it is hard to separate out the fact that such pages tend to get links and have high Keyword density from the authority of the site.
A relevant article that tries to refer to the above article is here. Unfortunately, while their article discuss link building "authoritatively," their link to the other article leads to a 404 error page - the URL is wrong. But it is still a good article, that discusses many of the errors people make, such as not linking to authoritative Web sites, avoiding links from 0 PageRank Web sites and so on. There are good reasons why people make those mistakes - they are all due to Search Engine Optimization (SEO) Superstitions, such as never link to a site with lower rank than yours, or do not link to sites unless they link to you (see SEO penalty for unreciprocated links).
Google quirks are best understood if you also understand the philosophy and approach of the inventors. What are Sergei Brin and Larry Page and their disciples trying to achieve and how do they think they will do it. That also tells you what Google is likely to do in the next year or two, as well as what they have done in the past. That is why I have included their original Stanford university accounts of pagerank and of the prototype Google search engine - with an extensive discussion