A good critic acknowledges the influences that had an impact on whatever it is they are critiquing. They assess the quality of the material with relation to other attempts of the same calibre. For example, one would not compare Stephenie Meyer's Twilight saga with that of classical literature, such as War and Peace.
Another quality of a good critic is less quantifiable. As with any good writing, style is very important. A mediocre critique is made much more palatable if amusing figures of speech are used.