On Page:
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Architecture: 301's, other redirects where necessary, www vs non www resolve, page speed, duplicate content, etc.
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Keyword Optimization: Title, Meta Description, H1, Other Alt Tags, Don't ever use Meta Keywords Tag, Body Copy, URL
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User Experience: If the appearance of your site sucks and no one wants to look at it, the rest is sort of pointless
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Effective Call to Action: Call to action / Conversion / Whatever anyone wants to call it, if you have one, is it effectively placed and designed, and is it "converting properly"?
Off Page:
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Links: Links are what build and drive a site. Without them, you will usually be invisible. How you structure them, acquire them, the distribution and diversity of what type....that's all anyone's guess as to what's best. Main point being, you need
some links.
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Social: Social
doesn't replace links, you still need traditional links. Social can create "buzz", show that people are talking about you and want to interact with you. There is some SEO value in this. Again, how much, and does it necessarily apply to every type of site or business? Anyone's guess. For most, there's usually some sort of SEO value in having an active social profile.
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Relationships: aka Guest Posting, Industry Partners, etc. Depending on the type of site and subject matter, for many, developing industry relationships and social relationships can be a good thing. It's more people to potentially talk about and interact with you. The more "buzz", the better. Again, for most types of sites. There's always exceptions, and this may not always apply, just like everything else.
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Time Management: Okay, we're not even at 5 for each of these, but whatever.

Because offsite can waste so much of a person's time, doing repetitive work that may result in little if any gain, we need to keep in mind working smart vs. working hard. There are tons of SEO tools, aggregators, and automation tools that will allow you to be considerably more efficient. USE these. Say your client is very socially oriented. Don't just waste all your time building social activity through a handful of manually created and manually built personas. Use an aggregator like Hootsuite, or buy BuzzBundle (or Buzzstream, whatever that SEO Powersuite tools is called) and plugin all of your personas to start doing things in volume and much more quickly. Work smart, not hard, otherwise the "best quality and most relevant content" in the world (that all of the "gurus" constantly go on about) may not help you much when your competitor is someone more efficient, faster, and able to do more work than you in a shorter period of time.
Kind of holistic but, the point here is, I think it's good to take a step back and look at the big picture sometimes. It's too easy to get caught up in HTML code and fine details, all the individual little on/off page SEO factors, and completely forget what the general game plan is or should be.