Here's a thought - All of our SEO and any non-original content is simply ignored. Google seems to be sectional targeting and applying off-site influences on the sections it deems unique. I have a feeling we're ranking only portions of pages lately. To prove that I've found pages that when I quote a section in Google search it returns nothing for my site but a different section ON THE SAME PAGE does return my site.
If you have a product feed for example from a popular retailer and you use it verbatim I guarantee you will not receive any traffic unless you offer more regardless of your SEO efforts.
More may include a review or images by the site owner, comments and 3rd party added content doesn't get full value.
Outside influences may range from your social net imprint to traditional incoming links to you as a profiled webmaster, probably all of the above.
For 2011 unique content may be the only way to go and expecting that to come from visitor comments and such may not be all you need to do. Also, it seems we are bound by expectations. Our sites are expected to attract a set number of people that have x profile associated with them. If we suddenly attract more, or less, a dampening factor feels like it kicks in. Watch your non-paid traffic levels when you start up your next major ad campaign to see what I mean.
Changes that increase repeat visits while lowering bounce rates are raising traffic levels on my sites nicely right now.
Fact: Google wants to rank unique content equally/fairly regardless of on page SEO. Google also places little trust in webmaster supplied information. I think they now have the technology to more closely accomplish those goals rendering SEO efforts less effective.
The big 5 in my opinion, and perhaps the only 5 factors that matter moving forward, are.
Unique and meaningful content
Page code (well formed, cross browser compatible, efficient)
Visitor behavior (relatively new to SEO)
I think Google is still working out the Visitor Behavior aspect of SEO because not all types of sites can be expected to receive the same reaction. Ask yourself this - Do MY visitors click on more pages and talk about my pages in their social profiles (and return more often, and leave more comments on etc) on MY site when compared to the sites Google deems related to mine? If not, anything you can do to fix that is actually SEO. With Google profiling visitors it's more important than ever.