I have been studying the effects of panda ever since april 11. I have reviewed many sites that were negatively impacted and I have even managed to recover a few. I have noticed many trends.
You may have been hit by panda if you do or have done the following:
1. Own one or more similar sites which all link back to your "main" business. We know that JCP was hit when google found out they had many sites which all link back to JCP. If you own one or more similar sites, its best to follow good practice and take them down or combine them to your main site. I have seen several webmasters get hit for multiple sites ranking for the same/similar terms.
2. Content placement. This falls is line with my ads theory. The location of your content is becoming a very real factor to google. This is backed up by googles instant previews in the SERP pages which outlines where exactly the text is found. If someone searches for awesome widgets, and you have the words awesome + widgets way down on your page, google might knock you back because that content will not be easy to find by the searcher.
3. Content stuffing. A very strong characteristic of a MFA (made for adsense site). Some of us cant help the temptation to try to fit all the keywords we are targeting into one small paragraph or few sentences. Write your content for your visitors, not google. In this case, some de-optimization is better than optimization. You can still rank for terms without having certain strong keywords found on your page.
4. Giving priority placement to ads over content.. This is a big one. Browsing the google webmaster forum so many people complain they were hit and have no idea why. Then you visit there site, click an article and the article is pushed all the way down by a huge adsense block. This tells the visitor "click my ads" instead of "read my content". If your site is optimized for ad clicks rather than for quality content, you may very well have a problem.
5. Over optimization and repetition. Panda is simply an update to googles already highly complex algorithm. Over optimization is an old factor but it is always being updated and panda is no exception. Google is getting better at sniffing out webmasters who blatantly try to game there system and they do not like it. Do not put your main keywords all over the place (titles, alt text, header tags etc etc) focus on having appropriate content.
6. Too many useless pages. Do you have a ton of useless pages with little to no useless content? Google may very well see this as an attempt to rank for many premium search phrases. If you have questionable content or complete sections of content that offer little to no actual value to the reader, it is probably best to delete/block it. Do you have a user based website? Do your member profiles all really need to be in googles index? Do you have tons of useless tag pages?
It is important to have someone that is not biased review your site. So many websites I have reviewed the owner swears up and down he/she has no idea why there "extremely high quality and unique website" was hit, then when I take a look I can spot many suspicious practices almost instantly.
Google panda is not entirely about duplicate content, if that were the case, all of these scraper scripts would not still be ranking.
I hope this helps push many people in the right direction. If you feel this information has helped you, please respond and if you recover, please report back with full details.