Matt Cutts in the latest Webmaster video addresses the hidden content issue and if it looks spammy to Google if most of the text is present in this section.
The latest video is an answer to the question posed by Thomas Greiner:
"How does Google treat hidden content which becomes visible when clicking a button? Does it look spammy if most of the text is in such a section? (e.g. simple page to buy something and “show details” button which reveals a lot of information about it)".
Matt said that he won't be overly concerned about the hidden content but there are different consequences, which need to be discussed. "Click here", "show reviews", "show specifications" are pretty common idioms found on the web and these are not deceptive. Cutts says that it is pretty visible that the hidden text is intended to give additional information to the users.
But, in case you have stored six pages content with no relevant information for the readers but with the intention to stuff keywords in the little button, Google might consider it as a hidden text. Cutts also said that in case you have a nice AJAX sort of a clean site, Google won't be worried about the website as there are a number of other sites who are doing the same.
Citing example of Wikipedia, Cutts said that Wikipedia on mobile has different sections, which expand when you click on them. There are good usability reasons behind the feature. As long as you are not trying to hide something deceptive or trying to play with rankings and are focused on delivering useful content to users, you will be in a good shape.Matt Cutts on How Google Treat the Hidden Content?,