TechDirt reports users notes and highlights you take on the Amazon Kindle aren't private, these pieces of text are uploaded to Amazon, which it then compiles into a list of "popular" highlights".
I have no doubt that the feature provides some interesting data, but it's not clear that users realize their highlighting and notes are being stored and used that way. Amazon basically says there's no big privacy deal here, because the data is always aggregated. But it sounds like many users don't realize this is happening at all. Amazon says people can find out they added this feature by reading "forum posts and help pages" -- but it's not clear how many people actually do read those things. While I'm sure many people are fine with this, others might not be. And it once again highlights a key concern in that the "features" of your "book" can change over time.