Here is some things that I've saw on my own sites, and other sites I know that moved places.
Possible reasons for falling: (I've seen these things in common on most that have fallen)
- Sites with rss fed content, or spun content. Unique sites mostly won on this update UNLESS they had more fluff above the fold taking away from their content).
- Heavy advertised sites, or sites with heavy ads above the fold.
- Sites with the same content above the fold on most pages (ie, large nav menus, large ad blocks, large 'recent posts' blocks etc).
- Sites with weak or mostly sidebar/footer inbound links (This would be obvious, as sites are devalued the link power they push is also devalued, so the links that had you ranking before, now aren't enough).
- Sites that overly stuffed or pushed for keywords. Where I've noticed this it was usually pretty evident that it was 'spammy'.
- Sites that have poor navigation, or are 'messy' to the bots. Today's web is user friendly, this why bounce, time on site etc is popular.
- Sites with little or NO 'buzz' about them, if people aren't talking about you somewhere, something is wrong, google sees this easily spidering the social arena. (Color lightened as I'm not positive on this on, but follows suit with what I'm seeing so far)
- Sites with a strong percentage of inbound links from sites like above, will cause you to fall also.
It's been pretty widely accepted that this was a massive content/linkfarm update, and these observations would help explain why good sites are being dumped as well, it's the way the content is being displayed. Have some quality stuff above the fold, push huge nav menus and add banks BELOW the content.
I'll be trying a couple simple fixes on some of my sites that disappeared this week, but am curious to hear other opinions.