I've asked my hired SEO gun. What he speculated is that Google is beginning to tighten up the discrepancy between on-page and off-page factor after the May's algorithmic update. This is just the beginning...
One of the issues is the relevancy of content and its link, because the update wasn't done by Matt Cutt's team. Instead, it was by Quality Search team. Perhaps the reason that it's easier to kill inaccurate the long tailed search first is due to the fact that most SEO specialists are attacking short search terms.
Nobody knows how Matt defines "high quality" sites. My understanding is that this could be the quality and relevancy of content and the links (incoming and outgoing).
Why I use this word 'and'? Because it seems that Google has found a way to relate both on-page and off-page factors. For example, Google may want relevant incoming and outgoing links to/from a content page. Google may also look at the relevancy of the neighbors.
We all know that relevancy is one big thing to tackle with. It requires much computing power to resolve this. I think Google still has problem dealing with this. That's why link-pushing is still somehow effective for the next.... 6 months? 12 months??
There is a publication regarding the May change:
Google Confirms “Mayday” Update Impacts Long Tail Traffic
Pay special attention to the additional note on 5/30/10.
Here is my experience...
I've a shopping site that relies too much on link-building via articles. The ranking stuck at number 15-20 for about a month no matter how much links I built.
So I hired a SEO guy. He did nothing but built highly optimized 'thick' content. And my site jumped to top-3 spot within 12 hours.
I should have done it by myself!
Should you accept this assumption?
It's up to you.
Nonetheless let this update be a warning sign.
We may not see the full effects. If the May update has changed the scape of long tailed search, it doesn't mean that Google won't deal with short search terms in the future. It's better to prepare for it than be sorry later.
Any input would be appreciated. Thanks.