We had a site hit by Panda-2 in April 2011. The content was good, the appearance was dated. We made purely cosmetic changes. With a more modern look we saw a quick recovery.
The site I referred to above was again looking rather dated, although less so than the first site that was hit. Again we made cosmetic changes to its appearance and this time we also improved navigation, which had been rather weak. This time we had to wait longer for recovery, which I hope won't be short-term!
In both cases, we made no changes to core site content, which we believe to be strong.