16. A system for annotating content based upon user reaction data, comprising: a first sensor that selectively detects user reaction data; a detection component configured to detect first user reaction data associated with a first portion of the content by detecting a presence of metadata of the content that specifies a probability of a user reaction to the first portion of the content, determining whether the metadata indicates that there is a probability of a user reaction to the first portion of that content that exceeds a threshold, and utilizing the first sensor to detect the first user reaction data, after the determining and when a result of the determining is affirmative; and an annotation component configured to annotate the first portion of content with a first reaction annotation based upon the first user reaction data.
16. A system for annotating content based upon user reaction data, comprising: a first sensor that selectively detects user reaction data; a detection component configured to detect first user reaction data associated with a first portion of the content by detecting a presence of metadata of the content that specifies a probability of a user reaction to the first portion of the content, determining whether the metadata indicates that there is a probability of a user reaction to the first portion of that content that exceeds a threshold, and utilizing the first sensor to detect the first user reaction data, after the determining and when a result of the determining is affirmative; and an annotation component configured to annotate the first portion of content with a first reaction annotation based upon the first user reaction data. 18. The system of claim 16 , wherein the detection component is configured to selectively activate or deactivate the sensor to detect the first user reaction data based upon metadata associated with the first portion of the content.