Notes in the margin
Amazon displays the most-highlighted passages from Kindle readers. I love it! Although I wish there was a way to slice-and-dice by genre, or look at one book or author specifically.
Update: Whoahdang! Zach Seward points out that you can, in fact, do exactly this. Here’s Philip Pullman. Here’s, heh, Robin Sloan.

There is! Just click on the titles or authors.
Woah, this is the Kindle equivalent of running a Twitter search right after you’ve watched a TV show to see if other people had the same gasp/chuckle/OMG reaction to certain scenes.
[…] from whatever book you’re reading at the time. Similarly, Snarkmarket revealed that you can browse the most-highlighted passages on the Kindle […]