1) Keyword-rich URLs and filenames: A very important factor, especially for Yahoo! and Bing.
2) Site Accessibility: Another fundamental issue, which that is often neglected. If the site (or separate pages) is inaccessible because of broken links, 404 errors, password-protected areas and other similar reasons, then the site simply can’t be indexed. This is not good as your primary goal with SEO is to get your site indexed thoroughly
3) Sitemap: It is great to have a complete and up-to-date sitemap, spiders love it, no matter if it is a plain old HTML sitemap or the special Google sitemap format.
4) Site size: Spiders love large sites, so generally it is the bigger, the better. However, big sites become user-unfriendly and difficult to navigate, so sometimes it makes sense to separate a big site into a couple of smaller ones. On the other hand, there are hardly sites that are penalized because they are 10,000+ pages, so don’t split your size in pieces only because it is getting larger and larger.
5) Site age: Similarly to wine, older sites are respected more. The idea is that an old, established site is more trustworthy (they have been around and are here to stay) than a new site that has just popped-up and might soon disappear.
6) Site theme: It is not only keywords in URLs and on page that matter. The site theme is even more important for good ranking because when the site fits into one theme, this boosts the rankings of all its pages that are related to your theme.
7) File Location on Site: File location is important and files that are located in the root directory or near it tend to rank better than files that are buried 5 or more levels below.
8) Domains versus subdomains, separate domains: Having a separate domain is better – i.e. instead of having yourdomain.blogspot.com, register a separate yourdomain.com domain.
9) Top-level domains (TLDs): Not all TLDs are equal. There are TLDs that are better than others. For instance, the most popular TLD – .com – is much better than .ws, .biz, or .info domains but (all equal) nothing beats an old .edu or .org domain.
10) Hyphens in URLs: Hyphens between the words in an URL increase readability and help with SEO rankings. This applies both to hyphens in domain names and in the rest of the URL.
11) URL length: Generally doesn’t matter but if it is a very long URL, this starts to look a little “spammy”, so avoid having more than 10 words in the URL (3 or 4 for the domain name itself and 6 or 7 for the rest of address is pretty good).
12) IP address: Could matter only for shared hosting or when a site is hosted with a free hosting provider, when the IP or the whole C-class of IP addresses is blacklisted due to spamming or other illegal practices.
13) Adsense will boost your ranking: Adsense is not related in any way to SEO ranking. Google will definitely not give you a ranking bonus because of hosting Adsense ads. Adsense might boost your income but this has nothing to do with your search rankings.
14) Adwords will boost your ranking: Similarly to Adsense, Adwords has nothing to do with your search rankings. Adwords will bring more traffic to your site but again, this will not affect your rankings whatsoever.
15) Hosting downtime: Hosting downtime is directly related to accessibility because if a site is frequently down, it can’t be indexed. But in practice this is a factor only if your hosting provider is really unreliable and has less than 97-98% uptime.
16) Dynamic URLs: Spiders prefer static URLs, though you will see many dynamic pages on top positions. Long dynamic URLs (over 100 characters) are really bad and in any case you’d better use a tool to rewrite dynamic URLs in something more human- and SEO-friendly.
17) Session IDs: This is even worse than dynamic URLs. Don’t use session IDs for information that you’d like to be indexed by spiders.
18) Bans in robots.txt: If indexing of a considerable portion of the site is banned, this is likely to affect the nonbanned part as well because spiders will come less frequently to a “noindex” site.
19) Redirects (301 and 302): When not applied properly, redirects can hurt a lot – the target page might not open, or worse – a redirect can be regarded as a black hat technique, when the visitor is immediately taken to a different page.