1. A method of annotating content based upon user reaction data, comprising: detecting first user reaction data associated with a first portion of the content, the detecting including firstly detecting a presence of metadata of the content that specifies a probability of a user reaction to the first portion of the content, secondly, after the detecting, determining whether the probability of a user reaction to the first portion of that content exceeds a threshold, and thirdly, after the detecting and a result of the determining is affirmative, utilizing a first sensor to detect the first user reaction data, so that the first sensor is used to detect the first user reaction data only after a result of the determining is affirmative; and annotating the first portion of the content with a first reaction annotation based upon the first user reaction data.