Just to follow-on from my last post, apparrently Google is working on a fix 

We got hit at StoreCrowd by around ~40%, whilst it's not devastating for our business it does raise some concerns about what we're doing (in comparison to competitors).

I've made a few observations:

1. Many of the websites that I've seen hit appear to have a large number of indexed pages (TeachStreet you have over 1M+ for example)
2. It appears to have hit sites that don't have a high amount of unique content to page ratio. For example we currently have a lot of blank "placeholder pages" & we also split out our merchants into 4 areas (store, coupons, deals, reviews).
3. The penalty appears to be sitewide, we have a large number of links & quality on our blog but even these pages have been hit hard.
4. The dropoffs in rankings can be a few spots or ~30 spots, I can't explain why this is - for less competitive keywords the dropoff appears to be less. This leads me to think this ain't a penalty but Google is simply reranking based on new factors.
5. I don't think this has anything to do with links.

What we've done so far:

1. We're working on increasing the unique content to page ratio - we've noindex,nofollowed all pages that have 0 content (or the placeholder pages I spoke about)
2. The next step is to further increase the uniqueness of our tag pages & store level pages.

But, we have competitors that have a lot more duplicate content than we do & they seem to be fine. So, we're merely testing & speculating at the mo!

Happy to hear any suggestions.