1. A computer-implemented method to identify training documents satisfying a predefined set of suitability criteria for training a content classifier to classify documents as belonging to a category of a plurality of categories, the method comprising: receiving a plurality of documents; identifying, based on the predefined set of suitability criteria and by an application executing on one or more computer processors, a subset of the received plurality of documents as being suitable for training the content classifier to classify documents as belonging to the category, wherein one or more documents in the plurality of documents is excluded from the subset, wherein the application is configured to identify the subset based on any predefined set of suitability criteria selected from at least three of a predefined content rating threshold, a predefined set of suitable user-supplied tags, a predefined viewing frequency threshold, a predefined set of link structure criteria, and a predefined set of metadata criteria; and training the content classifier using each identified document as a positive training example representing the category, wherein the content classifier is not trained using said one or more documents excluded from the identified subset.