1. A method for evaluating document quality, the method comprising: for a first plurality of documents, performing for each document of the first plurality of documents: identifying, by a computer system, quality attributes associated with the each document of the first plurality of documents, the quality attributes including characterizations of usage of the each document of the first plurality of documents, relevance of text in the each document of the first plurality of documents to one or more topics, and quantity of text and media; associating, by the computer system, a classifier from a plurality of classifiers with the each document of the first plurality of documents in accordance with relevance of content of the each document of the first plurality of documents to the classifier, by selecting the classifier from a taxonomy of concepts as being representative of the each document of the first plurality of documents by evaluating meanings of textual representations within the each document of the first plurality of documents; and receiving, by the computer system, a ranking of the each document of the first plurality of documents; training a plurality of class-specific models each corresponding to a classifier of the plurality of classifiers by, for each classifier of the plurality of classifiers, training, by the computer system, a class-specific model of the plurality of class-specific models corresponding to the each classifier of the plurality of classifiers according to the ranking of the first plurality of documents associated with the each classifier of the plurality of classifiers and both of the quality attributes associated with documents of the first plurality of documents associated with the each classifier of the plurality of classifiers and the content of the documents of the first plurality of documents associated with the each classifier of the plurality of classifiers; and for a second plurality of documents, performing, by the computer system, for each document of the second plurality of documents: identifying quality attributes associated with the each document of the second plurality of documents; associating a classifier with the each document of the second plurality of documents in accordance with relevance of content of the each document of the second plurality of documents to the classifier; inputting, by the computer system, the each document of the second plurality of documents to a selected class-specific model of the plurality of class-specific models corresponding to the classifier associated with the each document of the second plurality of documents; ranking, by the computer system, each document of the second plurality of documents according to the selected class-specific model using as inputs both of the quality attributes and the content of the each document of the second plurality of documents.