The most important factors right now seem to be:
- Importance of the domain; My guess is that this is mostly based on the sheer PR
of the domain.
- On-domain SEO. Pages with lots of internal links will rank high in the SERPs.
They will rank high for whatever anchor text is pointing to them, what's in the title, (possibly what's in the meta tags), and what's in H1.
- Quality links. These need to be high PR
/importance links. Links from important domains but low PR
do not help much.
- Anchor text of off-page links. This matters less now. The Googlebomb "miserable failure" isn't doing so well.
- The webpage isn't doing anything Google deems to be nefarious. Obvious cloaking (Google doesn't catch a lot of cloaking though), doorway pages, selling links, linking to bad neighborhoods.
- Quantity of links. A 1000 low-quality links from one domain will be far less helpful than 1 link from that domain's most important page.
Doesn't seem to matter much:
Topic of the domain. Wikipedia is about everything and not focused on a specific topic, yet ranks well for everything. It's hard to find a wikipedia page that is not in the top #10.
- Link Relevance. Are the linking sites of the same/similar topic; are they relevant to each other?
(Link farms will be very high relevance so this might be a reason not to value this.)
is delayed. The displayed PR
may not be the current PR